A lot of you guys are missing out the transmission mounts... It's an old car and if the tranny mounts have never been replaced before, you have to... Worn tranny mounts causes the transmission to be unstable when the car is running; the engine causes the tranny to vibrate vigorously especially at idle, during inconsistent amount of power delivery, as well as everytime you compress and depress on the clutch pedal... Chances are the tranny itself isn't aligned, which makes it eventually hard to shift to a certain gear/s...

Also, have the gear oil replaced... there are debris that circulate all over your tranny as the fluid does, so get rid of them...

btw, you can check for gear oil leaks by looking underneath, check on the drain and filler holes, and little possibly on tranny ends...

Also, have the cables, hoses, brake master/slave cylinder, etc, checked by a reputable shop/mechanic...