Transmission Popping out of Gear

A little backstory:
I've got an '89 Max, G63B with a KM132 5spd. I've done quite a bit of work to it, but that's a story for another day.

I've been plagued with transmission issues for awhile now. The original transmission shredded 3rd gear on the mainshaft, I was able to find a replacement gear and did a full rebuilt. The mainshaft had some bad wear on the input shaft/4th bearing, I didn't have the time to get it repaired so I put it back together and it started making metal confetti (It's a party!). I purchased a spare G63B and KM132 out of a Ram 50, and recently threw the transmission in after rebuilding it as well. Now that transmission is having issues, namely popping out of third gear.

Symptoms:
When driving regularly, it has no issues. When I jump on the gas (WOT) the shifter in the cab jumps about an inch at the knob towards neutral. It doesn't always pop out of gear, but if I jump on the throttle a couple times in quick succession it will pop out almost every time. It does something similar in fourth, but nowhere near as exaggerated. It has not popped out of fourth gear (yet). I've taken to skipping third gear, to try and reduce the risk of damaging something, but I have no idea what could be causing this. 1/2/5/R all behave as they should, no issues. I tried turning the detent screws in further, which made shifts firmer, but did nothing to reduce the issue.

Any ideas?

My current plan is to rebuild the original tranny with a new mainshaft. I have a KM132 out of a Starion (I've got a 4g63t and plans for someday, so I needed the tailhousing) and am planning on swapping that mainshaft into the original transmission. That way I can at least get the truck back on the road. I would still like to get this one (The Ram50 transmission) working if I can, as a backup, since I keep having issues with them. Any input is appreciated! Thanks