Manual valve body 46re 3rd gear issues :(

46re with transgo shift kit to convert it to a full manual vb. lockup and overdrive are activated by switches that ground the solenoid.

This is in my mopar 360 la powered 88 wrangler on 39.5" tires, with steel beadlock wheels. So very heavy.

Was at the sand dunes few weekends ago and took 10 or so passes down the 1/8 mile strip. Tires aired down to 5 psi, 4 low, and shifting through all the gears reaching around 60 mph.

Driving around town, it momentarily would lock up the rear end and release. Very scary. Did this twice in a row, then not again. Other day it did it again...however I was going slow and it felt like it just downshifted to second. I was going fast when it first happened so I think the downshift caused the rear to stop suddenly.

This is a rebuilt trans with around 800 miles on it....all very very very hard miles.

Line pressure is 130ish, when lockup and od on, it's around 150. Line pressure does not drop when it has the issue.

What should check for? Seems it happens when trans is not cold