I'm hoping there's some relationship to clutches in general, and 
maybe other Jetta owners have had this problem...

On my '85 TD Jetta with 371k:
The last 3-4 weeks, my clutch has started slipping in 4th & 5th gears 
while climbing long steep hills, usually only after pushing hard to 
pass. This week the threshold of slipping has dropped below about 
90km/h, making me wonder how long I can put this repair off... :(

I replaced the original clutch/press plate at 250k, although it still 
had lots of disk left, it was soaked in oil due to leaky crank seal. 
(It didn't slip then, just chattered).
When I put in the TD engine at 280k, I forgot to check/replace the 
crank seal, which I suspect is now leaking, as all inline VWs seem 
to. Another possibility I've thought about is the flywheel has worn 
beyond its usable life and cannot create the pressure needed on the 
clutch disk.

I'm assuming it's the oil leak, but can flywheels wear out at 371k 
km? I might tackle it this w/e, just wondering what I might find in 
advance...

tia