coolant woes

[Luke P <lukep@ihug.co.nz>]

Somebody may have btdt and be able to help me out.

The vehicle in question is an 88 4cyl 80.

Recently I ran some Prestone radiator cleaner thru the car as the
previous owners had sadly neglected the poor old girl.
I adhered closely to the instructions and ran the car for about 6 hours
with it in. I then drained the cooling system totally and added pure
water as per the instructions for 4 hours. I then refilled the system
with water and anti-freeze and went on my merry way. Today (1 week
later) the car overheated on the way up the road (2 km approx) 110 C
Flashing light etc etc. I let the car cool for a while then drove home
slowly. I turned the heater on to facilitate cooling and no heat came
out. But yes there was still water in the system, in fact it was nearly
full. When I got home I looked under the hood, all appeared normal
except the bottom radiator hose was stone cold. I Let the car cool then
started it up again and everything functioned as advertised. However
when I returned from my test drive a nice little geyser was erupting
from the expansion tank.

I have several theories.

1.    Flushing removed so much s**t from the system that it all
collected to block the thermostat which is conveniently placed at the
lowest point in the system. And a coincedental failure of the coolant
resevoir cap

2.    The combination temperature sensor had magically failed and set
off the light and gave a high gauge reading, it then repaired itself
when I got home (but the fuel injection was working fine) and the car
hadn't actually warmed up enough to open the thermostat (it is summer
here)

3.     Water pump has s**t itself and is only just providing enough flow
to cool the engine.

4.    Head gasket has gone (please god don't let this be it)

Any ideas

TIA

Luke