Installed autometer guages, couple of questions.

What wire did you tap into?? i know its supposed to be all over the place because you bypass the antislosh module but not when you rev the car. I would double check your source wire.

so the fuel guage is acting like a tach??? haha... i don't know man, i just thought it sounded funny

im not at all familiar with the aftermarket units, but ill toss out what comes to mind.

as said, you have bypassed the slosh module. the alternator ouput probably messes it up too. without something like the antislosh module, im not sure how to attack the problem. someone like Tom or Joe might know.
it is common for the reading on aftermarket gauges to bounce around while driving w/o the module (even w/o alt changes). said another way, other autmeter-fuel-level gauge users have said the same thing.

i would just look at the gauge while idling on flat ground with the rpms up a bit, to where you have decent voltage (i look at mine about once a drive). also, you might notice more variation than you do even now, at night with the lights and the blower motor on.....
my lame thoughts.
good luck bud.