Variously not working...Fisher 7'6" SD 10 years old

Hit wrong key and didn't get to finish question. Sorry
Went to use plow. Hit up. Nothing. Hit left button - it goes to the right and stays there. Wont go up or down. Tried pushing it into a bank a few times to get plow up to go to town and get machanic to look at it. After pushing piston a few times to the left all of a sudden the controls work again.
Mechanc changed solenoid under hood. Next dday same thing.
Mechanic changes the hydraulic fluid and cleans the filter. He mentions a lot of corrosion on 3 pin connector going to the plow but he doesnt want to keep just running up my bill by changing alot of parts.
Next day same thing. So I buy an new wire harness that goes to the valves on plow. It works fine.
Next day not working again.
Mechanic replaces valve that makes it go left ( i think) says it was not working and that might make all the valves act strangely.
Next day not working - plow goes left and right and up but won't come down. This is different from the last few times it wouldn't work.
I decide I'm going to replace the other 2 valves myself but then have an idea to throw a hairdyer on the pump just above where the valves are mounted.
After 5 minutes of heat the plow works fine again.
Pulling my hair out. Decide not to replace the valves. Plow my yard and the neighbors no problem and decide to see what happens tommorrow ,Sunday.
I might mention everytime the plow did not work the temps were in the 20's like this morning. My imagniation says when it was above 30's its been working fine the times before.
What have we missed??

Update
Brought in to the mechanic that has been working w me. Took off cover from pump solenoids and found no power going to the magnet? Said he pulled on the wire harness under hood which is run tightly in fron of the battery and suddenly the plow went down. He thinks worn wire shorting maybe but electrical is not his expertise. Thought maybe me using the blowdryer on pump was a fluke fix.
Its strange that the original problem I brought this in for is different from what is happening now. As far as electrical possiblility i recall that I changed the harness going to the pump on the plow side; the 3 pin harness - could that be defective from factory?
Contacted two other plow machanics. One can't get me in til Thursday mornng to look at it. Second company said maybe new harness bad but the problem would have ot be consistent to check it out- they also said they had seen strange things happen w Fishstick controller going bad and said maybe try to swap it.
This is getting very frustrating. Intermittant problem is the worst. Since he touched it yesterday it had worked fine plowing the rest of the day.
Will try again later today.

SnoFarmer said:

Pics of the under hood harness?

see any spices or electrical tape?

is the solenoid grounded with good connections.

What does the inside of the plug at the grill look like?

Click to expand...

So much crap in there and they have it run so tight to the battery not sure I could get a pic if I knew how to post one.
No obvious wear marks . Zero tape or splices. Actually looks brand new like day they installed it.
New solenoid as of last week and everything is clean and shiny an tight.
Plug at grill is clean. This the 3 pin plug I'm talking about that goes to the pump. Checked w meter an I have voltage on truck side pins. I'm thinking its the single pin by itself that goes to the white wire that controls the down control of pump? Hard to check by myself but it seemed at first there was no voltage at white wire on pump but there def was voltage on truck side.
I'm just thinking that this new trouble started after I replaced the harness last week going to the pump while trying to figure out the original problem I had last week.
Bad harness from the manufacturer?
Of course after messing around and checking voltages the f--kin thing started working suddenly. Once it starts working it has continued working for the day so won't know again until tommorrow.
Really appreciate your help. Thank s much.

To upload a picture click on the icon to the left of the camera, click on the window that pops up
Then the next ont too and then select your photos from the next window that pops up

cwren2472 said:

Incidentally, the chances of a new harness being defective are extremely low.

Click to expand...

I did not touch anything on the truck side of harness except to yank on the bundle of wires he said he touched. That side of the system is brand new last year when installed on my new truck.

I was checking voltages with plow side disconnected on the truckside plug. Had voltage hits on all three pins on that plug Through deduction I think the single pin goes to the white wire. Orignally with everything plugged in I was getting no voltage on white wire on pump. Of course I put the whole thing back togther to test voltage again and it was working!!
As a test I put the old harness back on and will try it again tomorrow because once it starts working its good for the day.
I hear what your saying about new harness unlikely bad but in my world of electronics I 've had a lot of stuff defective out of box chasing me down rabbit holes until I tried the old stuff.
Another tech on phone is telling me the coil to the valve might be bad - it seems like if it was I would still have voltage going to it but it wouldn't magnetize.