I used to have the same problem with my 1.7 cabrio after I replaced the
radiator. The problem turned out to be a pratically faulty aftermarket
thermo-radiator fan switch. It is located on yhe driver's side bottom
part of the radiator and screws into the radiator directly. It is about
1" dia. I got those pieces of rubbish at autozone, and they're junk,
made in some Tijuana alley. I replaced them twice, until I finally went
to VW (aargh) and got the original part. Once I installed that, it
stopped running hot, and the fan would come on earlier.
Check if it's running hot when you'redriving on the freeway (at higher
speeds), if it is, you might have some problem elsewhere.  Good Luck.

Choreboy