OT-Jetta running cold?

[rfkellner at snet.net (Rick Kellner)]

I have been preparing my 91 8v Jetta for the winter.  I know it is early
however; I do not want to be working on cars in December.  Now, the
Jetta had major plumbing problems.  First the radiator was leaking, the
heater core let loose and the thermostat seemed to be stuck open.  The
first two problems are easy to diagnose, the thermostat, or the symptoms
of the thermostat are proving to be less obvious.  
The car had been running cool, just barely above the solid mark on the
gauge.  Normally it would operate near the middle of the gauge.  So when
all the plumbing work started I decided to also replace the thermostat.
I even tested the new thermostat to make sure it opened.  Well,
everything is buttoned up with no leaks however the car will not heat up
to normal temps!  I know it is not the gauge because the air from the
HVAC system is not that hot, even in this weather.  Also, if I let the
car idle it will eventually heat up to normal temps and the cooling fan
will run.
Is it just my luck to have two thermostats that opening at lower temps?
I ordered the higher temp one since this is a digifiz car and they need
to run at temp for the FI to operate properly.
Any other ideas besides the thermostat?  
Thanks, Rick.