You've been around these machines longer than me, but since I've owned manual transmission cars for my entire life, I've found the following out. Slipping in and out of gear and not going into a geat smoothly is "normally" a worn or bent shifter linkage matter. Noises coming from the throw out bearing can seem to be coming from the tranny and I try to avoid my habit of keeping the clutch in at stop lights instead of shifting into neutral to save the throw out beaaring.
I've read many threads about not being able to rebuild these transmissions, but if it's made by man it can be disassembled and reassembled by man, but at what cost and with what capability is in question. I have seen M5 transmissions regularly on Ebay. Sometimes changing fluid can help and I found an oil change with Royal Purple made my transmission much smoother - although my wife thinks it was the placebo affect. If you think you have bearings whining or a missing tooth in your tranny then you may need another one. My son had a Fiero (don't laugh he loved it) with a gertag manual and the dealer told him the same thing, it couldn't be rebuilt and he'd have to get another one from Pontiac, but he took it to a transmission shop and they found a few missing teeth and got him a new one from an old parts car for one half the price of a new one or rebuilding it. They were up front with him telling him that the gertag manuals are labor intensive to rebuild and we all know the hourly cost of a qualified mechanic isn't cheap!
I've read many threads about not being able to rebuild these transmissions, but if it's made by man it can be disassembled and reassembled by man, but at what cost and with what capability is in question. I have seen M5 transmissions regularly on Ebay. Sometimes changing fluid can help and I found an oil change with Royal Purple made my transmission much smoother - although my wife thinks it was the placebo affect. If you think you have bearings whining or a missing tooth in your tranny then you may need another one. My son had a Fiero (don't laugh he loved it) with a gertag manual and the dealer told him the same thing, it couldn't be rebuilt and he'd have to get another one from Pontiac, but he took it to a transmission shop and they found a few missing teeth and got him a new one from an old parts car for one half the price of a new one or rebuilding it. They were up front with him telling him that the gertag manuals are labor intensive to rebuild and we all know the hourly cost of a qualified mechanic isn't cheap!