Oldsmobile Aurora Tachometer Problem

I have a 2001 Aurora. Recently when I have been driving, there is a slight pause in the engine, then the Tach jumps up between 5000 - 8000 rpm. I don't think the rpm's are really that high as it sounds normal. Any ideas what this could be?
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I just had it done - $164 - but my mechanic rocks and is not a thief.
Problem solved! And no more hesitation/missing on passing.
was it the crankshaft sensor?
I have the same problem
then tach jumps to 4-5x what it should read. motor stills runs fine most of time,
sometimes rough on restart, but smooths out & then ok. i will try the CPS, seems
to be a common problem on the 01 aurora 3.5