this problem is litteraly so common i didnt have to open this post to have a pretty damn good idea what it was about...if im not mistaken the 5th gear pop-out is caused by bearing play in the 5th gear shaft (most FWD transmissions put the 5th gear on a seperate shaft to save room)
this problem was corrected by 93 so the easiest thing to do is to change the trans out with either a JDM trans or a really really hard to find 93/94 trans... or a g20T trans (standard g20 didnt have the limited slip and the axles wont be the right length because of it... the Touring or T model did)
good luck to it... i had to take my 91 se-r off the road years ago cause i couldnt dig up a trans back then... and a jerkoff neighbor called the township to force me to get rid of the car sitting in the back of my driveway... i still wake up in the middle of the night crying for the loss of my poor white classic
