Background:

I bought an 8-valve 944 this summer for cheap and fixed it up slowly... the relevant repair was a new fuel pump (Regina, in-tank) and fuel filter. I just started driving it a couple of weeks ago. The car has almost 200,000 miles on it. It drives great, but...

Symptoms:

When I fill up with gas, the fuel gague sticks at a little more than half a tank for a while. Then (after going over a bump? after making a sharp turn?), the needle goes all the way up to full.

Later, when the needle is almost to the red, I buy gas, -- and I can only pump 9 gallons before the tank is full.

Theory:

I was under the impression that these early 940s have a 15-gallon tank, so the gas guage is under-reporting the contents of the tank dramatically when it's "empty".

Questions:

Has anyone else experienced this? Is it just a consequence of age increasing the resistance on the float electronics? Could this behavior be explained if I put the fuel pump/float mechanism back into the tank at a slight angle off vertical? Is there an easy way to fix it?
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'92 245 5-speed, '92 944 GL auto