This sort of behavior is typical of a plugged radiator or bad
(eroded) water pump. A bad fan clutch will cause high temps in low
speed situations like idling in traffic, and the electric fan will
come on at 100C or so. If it doesn't, fix that, too.
Heating under load is either a bad thermostat (failing to open
completely) or a clogged radiator or bad water pump as the radiator
is hot shedding the heat properly.
It is possible that you have a bad head gasket (or it wasn't
installed properly, etc), in which case you will have substantial
cold pressure in the radiator.
One more thing to check -- if you have had a leaking front seal at
any time, oil collects on the radiator and that causes dirt to plug
it. It's easy enough to pull the rad on that car, it's worth the
effort to pull it and empty it. Clogging of the fins is obvious, and
will be worst behind the hub of the electric fan. You don't have AC,
so you don't have one large trapping point -- the space between
condenser and radiator is very prone to collecting chopped insect
parts! No air flow will give you overheating under load, just like
no coolant flow.
Peter
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