I have a 95 Z34 Monte with high miles and an attitude problem. The engine revs up to around 2500 then dives to 200 or so, and will eventually stall and die after a few of these cycles. This only seems to happen after the engine runs for awhile. I've replaced the mass airflow sensor, the throttle position sensor and the idle air control valve. A friend mentioned a coolant temp sensor....Comments or suggestions appreciated.

may be a coolant temp sen. but i would also look for a vacum leak.

Pull the vacuum line out of the brake booster and plug it off. Start the car and see if the surging goes away..

It is most likely the coolant temp sensor, mine did the same thing but after i changed the intake gaskets and the temp sensor it runs like a charm.

change your intake gaskets, these are common with the 3.4's. also while you are in there change the oil plate gasket under the intake , it is not available aftermarket but the dealer sell it for $10.00 this is also a vacuum source. this will solve your problem.