4th (and sometimes second) popping out of gear when off the throttle

[iSTURBO]

SO....

Clutch in, all good it can handle the boost, BUT....

It's now got a bad habit of popping out of gear. It only pop out when my foot is off the throttle, If i'm on the gas it will hold itself in but I can feel it vibrating quite a lot through the gear knob.

I've noticed it do it in second a couple times but it usually doesn't. 4th is the main culprit and it's so bad that with the clutch in (Engine running) it won't stay in 4th if I don't hold it in gear, it just pops right back out.

Interestingly enough I have noticed that on the odd occasion I will put it in 4th and it doesn't vibrate so hard and in this instance it will hold itself in when not on the throttle. However this is a rare occurrence.

Of note it did do this before the new clutch and flywheel went in but nowhere near as bad, only popping out on occasion when coming off the gas on the overrun.

What could be causing this and is it likely to require a full strip down of the gearbox?

Gearbox is a getrag 260 on and e30 m42 motor.

[DesktopDave]

I'm not an expert on transmission troubles, but I'd start with the cheaper stuff first. Check your motor, transmission and diff mounts. If the tranny is moving around you'll get all sorts of unexpected behavior. Any odd clunks or vibrations? Check your guibo and CSB too.

If it's just 2nd and 4th, could be a worn out shift linkage. When those bushings wear down, the shift lever and rod don't work at 100%. That can result in partial gear engagement. I can recommend the UUC shifter...it's a work of art, but a bit pricey. New OEM bushings might do the trick.