Transmission pops out of 5th gear into neutral while driving

I bought a 92 3000GT SL with a blown engine - swapped it with a JDM engine from Japan. Replaced clutch while I had it apart. Runs great so far, excellent gas mileage but have only put on about 1000 miles since the swap. Took it on a 600 mile round trip this weekend and part way into the trip, was cruising along about 70-75 and the transmission popped out of 5th gear into neutral. Happened a few more times on the way but got progressively worse. On the way back, it would pop out very frequently - so much so that I had to hold the shifter forward the rest of the way. It doesn't take much pressure to keep it in gear but was getting tired of holding it by the time I got back. Is there any linkage adjustment that could be causing this or is the transmission starting to go. Didn't see any other posts with this problem. This is FWD only and don't know the history of the car. It has 122K miles on it. Any ideas?

you can check the shifter linkage to make sure its secure (beneath the air filter box, on top of trans), but i highly doubt that is the source of your problem. this is a common problem on high mileage FWD manual trannies for our cars. often, something in the 5th gear assembly has come loose and/or a part or two is missing or damaged.

if you check the linkage and everything look secure, i would then suggest you remove the end cover on the trans. this is easily done with the trans still in the car, and removing this cover will reveal the 5th/reverse gear assemblies. visual inspection should be enough to determine which parts are out of whack.

to access the tranny end cover, jack up the passenger front tire, remove tire, remove inner fender well plastic revealing the transmission. the end cover of the transmission (right in front of you) is held on by 7 bolts. remove the end cover and snap some pics, upload them here, and i can tell you what's out of whack.

here is the end cover
http://bmtranny.escic.com/images/000_0370.JPG

5th/reverse gears
http://i57.photobucket.com/albums/g236/Linebckr49/3000GT/DSC02437.jpg

http://i57.photobucket.com/albums/g236/Linebckr49/3000GT/DSC02438.jpg

:worshippy:worshippy:worshippy

damn, linebckr...thats all i have to say

Thanks for the detailed info Linebckr49. I have another project on the go right now so won't be able to check it out further for a while. I've been driving it to work the last couple of days and haven't had it pop out of gear at all. I travel about 50 miles round trip each day and about 1/2 of that is freeway driving (I do get up to 65 or 70 mph some days). It never happened before the weekend trip and hasn't happened since. If I do get some spare time, I will pull the cover and post the pics. Thanks again.