5kTQ overheating - need help

[Steve Sears]

Hello there listers,
My '87 5kTQ has been misbehaving in the temperature department of late, and
has really got me stumped.
About a week ago the car overheated on the highway, resulting in a tow to
the local dealer's place and a $140 thermostat/flush'n'fill/car wash - ugh.
Took it on short trips and no problems - in-dash gauge reading lower than
100C (solid).  Took a trip to my mom's on Saturday, about 100 km of 120 km/h
highway driving went well, but just after coasting through a town at low
revs (ie 60 km/h in 5th), the car overheated after accelerating back to 90
km/h.  Stopped for a while and then resumed travelling after topping
reservoir up - drove the rest of the way with the heat on and windows open -
gauge stayed around 100C.
On the return trip, the temp gauge had my full attention.  The 90 km/h
driving went well, but upon reaching the 120 km/h highway driving, the gauge
soon began to oscillate with increasing amplitude - from about 98 to 100,
back to about 98....102......99......103......100........105 (BEEP!!!!
warning light - pulled over).  Fan was running and could feel bubbling in
the upper rad hose.  Waited and the topped-up and restarted drive as a
test - stayed at about 90-95 km/h in 3rd of 4th gear (revs about 3k to 4k) -
coolant temp stayed about 100C, no oscillations, no increases except on
hills (but not a dramatic change, only about 2 degrees warmer) - drove at
this speed unless in traffic jams, where the car had no problem sitting
idle.
The archives have offered:
Clogged rad?  Wrong/failed t-stat?  Busted water pump?  Blown head gasket
(yikes)?
What say the collective?
Thanks,
Steve Sears
1987 5kTQ - hurtin' (heatin'?)
1980 5k - helpin' (got me to work)
1962 and '64 Auto Union DKW Junior deLuxes - oblivious
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