HOT running MC... sporadic fan. Help?

[only5kgt@uswest.net]

Hello again...
The 87 5kcst, recent recipient of a new 7-blade fan and new
hoses for the coolant, has begun yet another cooling
shenanigan. It had been working great (that new fan was a
lot quieter) for a few weeks- last night, I noticed that the
temp gauge had gone AWOL again (it works the majority of the
time- bad multi-function sensor, right?) so I felt the hood
as I hopped out. Really warm. No hot smell or anything, but
up there. Those of us with calibrated thumbs know. I
wondered why the nice new fan didn't come on. As if in
response, it did. For about five seconds. Then off again. I
shook my head and went inside to sleep.
Behaved fine on the way to work this morning, but that run
can sometimes leave it not hot enough to run the fan anyway.
Inconclusive. On the ride home, same symptoms. Got good and
warm (I know, the temp gauge will be fixed ASAP!) and about
five seconds of fan.

The obvious answer (from an outside perspective) would be
that the multi-function sensor turns the fan on, of course
it's not working. But I recall that this fan control is
modulated by input from the sensor under the lower rad hose.
And I've had the fan working just fine with the gauge AWOL
before. Resistor for fan speed? Do I need to replace that
radiator sensor again? The fan works fine when AC is
switched on, so I'm ruling out connection issues there
(rightly, I hope)

Thanks for input. The 5k, with its lukewarm AC and list of
weird issues like this, is starting to try my patience. Just
summer heat, I guess.
Regards,
Rob