rpm dropping, tach bouncing
 

I have a 2003 Blazer 4wd. In park and idling, the rpm lugged down and the vehicle started vibrating lightly, while the tach started bouncing all over the place. I gave it enough gas to get to about 1500 rpm and the vibration got stronger and the tach started bouncing around more violently. It was reading anywhere from <500 to about 6k. I shut the motor off and started it up, and it was fine. A few days and over 100 miles later with no problems, it started acting up again, but this time it died altogether. I got it started with trouble, and within a few seconds it shuddered and died again. The tach was jumping all over the place this time as well. I went through this about four times, and about half of those times it sort of just died away, while the other half it seized up more and made a squealing sound as though the belt were binding.
What could cause the tach to go crazy and the engine to die like that?
*After fighting with it for those 4 or so times when it was dying I got it moving to limp it home, but while driving with sustained higher rpm it behaved pretty smoothly.

Crank sensor may be hitting the reluctor?

Is the SES light on? Any DTC's in memory? Have you checked anything yet? Fuel delivery problem??? distributor cap & rotor???

Code reader says Crank position sensor. I haven't had a chance to look at the thing to see if it was hitting the reluctor wheel. They can also be tested with a multimeter, correct?

What DTC's are set in memory, and in what order were they retrieved?

The crankshaft position sensor is very sensitive. If it is moved, removed, or disturbed in any way, (including loosening the bolt) the crankshaft position sensor relearn MUST be performed, (requires a scan tool capable of initiating it).