Dave:
First, the clutch:
Lots of good info on this thread - pay attention to it.
Something else to consider: Once you take your clutch out, find a really good standard transmission guy in your area who knows clutches and works with them daily and take the disk and pressure plate over to him to look at. Are they discolored? Do they show abnormal wear (like more in one area than another)? When you've replaced the disk in the past, did you also replace the pressure plate or use the original one? If it is a "fingered" pressure plate, are all fingers engaging equally" Diaphram clutches usually don't have this problem, but I have seen some weird things happening even with diaphram-types.
The pressure plate may not be parallel to the flywheel, or it may have more pressure on one side than the others. This condition can cause clutch chatter as it's engaged - can feel a LOT like wheel hop on a VW, as there's a lot of vibration going on back there and you can't look around to see if the wheels are actually hopping. The un-parallelism can be caused by it being slightly off-center with one side (usually the bottom as you assemble it) perched up on the slight depression in the flywheel that it's supposed to sit in when you tightened it down - hard to notice, easy to miss as it'll torque down just fine but be out of round, maybe warped AND not parallel, all at the same time. When you torque the pressure plate down, do you go in three stages of 1/3, 2/3 and full torque and use a star-pattern for tightening? That way you get it to tighten evenly to the flywheel and not warp (you can also make sure it's centered on the first round of tightening).
If you've not replaced the pressure plate before, I would now, along with the throw-out bearing and clutch disk, AND re-check the torque setting of the Gland nut in the center of the flywheel - 300 ft. lbs and no less!
Also check for leaking seals on the tranny. A leaky crankshaft seal seldom gets the clutch disk wet (although anything's possible) and if the clutch disk gets wet from a leaky seal it'll slip like hell but it'll be obviously discolored (and usuall smell bad, too!)
Lastly, get a good VW shop manual and adjust your clutch/pedal freeplay to spec after the new clutch is installed, just to make sure it's right - that may cause premature clutch wear, too.
Popping out of gear: Before we get crazy and start pulling the tranny out, remove the inspection cover at the back of your center tunnel for access to the shift rod coupler at the back. Make sure it is in good shape, tight with no obvious breakage or wear. (For that matter, you might just want to replace it with a new one, and don't forget to lube the bushing inside the center tunnel holding up your shift rod while you're working there).
Then, place the tranny in that gear which it pops out of (I'll assume it's fourth gear for now), then GENTLY remove the entire gearshift lever, exposing the coupling shaft and the "cup" which the shift lever engages. Now try moving the shift rod further forward (still assuming we're in 4'th gear) to see if it moves (it shouldn't). If it moves slightly forward and stays there, then I'll bet that your gearshift lever was mounted too far to the rear, thereby not fully engaging 2nd and 4th gears. Move the lever assy forward a bit and tighten it down and see if that helps. It should be quite vertical when in Neutral, but I don't know what you have for a lever.
If it pops out of 1st or 3rd, then the lever may be mounted too far forward, and not letting it go fully into 1st and 3rd.
Now, I may be jumped on for having these lever-moving directions backward, but I think I've got them right.
I would start by checking out the rear coupling and moving the gearshift lever mount forward or back to see if that helps. If not, then a tranny rebuild may be in order. sorry |>(
gn