I would like to tear one of them down to see exactly what's failing, Maybe I can walk you through it???
The change in line pressure has me concerned, Using a DVOM set to AMP scale, Connect one of the leads to the each of the Pressure Control Solenoid wires.
Pin 6 of the Clear connector is the PC High control
Pin 16 of the clear connector is the PC Low control
Use your scanner to monitor the Commanded Amp's......It should match the Actual output read by the DVOM. If it's off (Especially higher Amp's).....The PCM is faulty or You have a bad ground.
These transmissions may be eating pumps for some reason.....Like engine/trans alignment issues, Are both dowels present & in good shape??
Flexplate runout?
Inspect the pump gears, pump bushing, & pump body for wear/damage.
Check the Boost Valve Sleeve for being loose in the bore, The will cause Torque Signal oil to leak & cause low line pressure when the unit is hot.
Same with a loose Pressure Regulator Valve.
4L80E Reverse Bands are not suitable for plowing duty, Use a TH400 "Grooved" Band.
Check the Forward Drum for damage where the Input Shaft & seals ride, VERY commonly over-looked & will cause repeat Forward Clutch failure.
Use Raybestos grooved HD frictions in commercial applications (Forward, Intermediate, Directs)
Do not use 4 frictions in the Intermediate Clutch, Use 3 TH400 frictions, Wave Plate, Steels & Backing Plate. The 4 friction set-up is great for holding power BUT no so much for a HD set-up.
Set your end-play correctly!!!!! TH400/4L80E will beat themselves to death with to much end-play, Very quickly in a plow truck that is shifted from forward to reverse a million more times than a regular truck.
http://www.sonnax.com/articles/408-p...-train-failure