Overheating '87 Scirocco 16V

[lwjanzen at hydro.mb.ca (Janzen, Lawrence)]

Well, it's been 10 years since my '87 Scirocco 16V has seen the road.
The problem is I can only drive it for about 20 minutes!  It appears to
be overheating.  The temperature gauge goes slowly climbs to the top and
the red dash light flashes starts to flash.   I did replaced the coolant
on the weekend and yesterday I replaced the thermostat, hoping that it
would solve the problem, but it is still overheating.  The original
thermostat was a 87C and I replaced it with an 88C.  The waterpump belt
is still the original but appears to be good.  Keep in mind it the car
sat for 10 years without being started.  Looking for suggestions as what
to check next.

Thanks...Larry