Try tapping on the gauge cluster when its acting up. Mine reads correctly most of the time, then sometimes drops to about half what normal is. I'm suspecting bad connection/corrosion at the back of the gauge.
As to factory gauges being inherently bad, thats a load of you know what..
First off, you want to talk about mass produced instruments, go to your local wal mart, and look at the thermometers. Same brand, same inside temperature, every single one of them will read differently. I counter that if you don't buy a top dollar aftermarket gauge, you have absolutely no clue that your new gauge reads correctly either! I know that *I* don't have any gauge testing apparatus.
As well, *in my eyes*, it has been proven that GM calibrated these gauges from the factory, for accurate reading. The ceramic pieces on the back of the guages, depending on color, do in fact have different resistance, and there are many different resistance ceramic pieces to be found. No reason for different pieces if GM didn't care, and wasn't tuning them.
Dorian
My tech/links page: <a target="_blank" href=http://www.dorianyeager.com/index2.html>
http://www.dorianyeager.com/index2.html</a>
No anti-theft measures on your truck? No pity when its stolen