Engine stalls at stops and inst cluster goes off w/headlite on

Could be a crankshaft sensor.or cam too

No, not the oxygen sensor. A couple of things you need to make clear. What light flickers, then stays on? That can't be a sensor light, it would have to be the "check engine light" or an engine temperature light (for like when your cluster has no temperature gauge, but just the "red light").
At any rate, the sensor you changed is not the engine temp. sending unit for the instrument panel. I believe on your '89 there would be two sensors for temp. One is the CTS for the engine computer, and the other is a sending unit for the dash, probably a 1 wire connector. It could be at fault, but it wouldn't cause stalling, I do not think anyway.
If it stalls and won't restart until cooled, this is a symptom of loss of ignition (spark) caused by either a faulty coil, or faulty Ignition Control Module (the coil controller), or last a faulty Crank Position Sensor (this sensor used by computer to determine ignition spark timing). You could test if ignition is lost, by checking for spark right after the engine warms and then stalls. If you have no spark to a spark plug right after it stalls, this is the reason for stalling. Usually it is the crank sensor, rather than the ignition control module or an ignition coil. Your car has 3 coils anyway in a "coil pack", so if one coil went bad, you'd still have two good coils, but engine would only be firing on four of the six cylinders-could easily stall a car.
You should check for trouble codes whenever you have a problem. A code might have been set. Easy to check for codes on your car, just ask if you don't know how-only takes a jumper wire or even a paper clip works. good luck.

Sounds like a TPS problem throttle position sensor

look around engine for any breather pipes ,they could be split and make sure you have a look and dont think i have already done it

My Pontiac Grand Prix is also a 99, it has 216,+ thousand miles on it, it runs great, idles good, I need to replace the oxygen sensor tho as it shuts off on me while driving or sometimes coming to a stop. You might try checking the fuel filter and pump as well.

do an internet search for "idle learn procedure" for your car and follow the steps. worked great for my 93 grand prix

RPM's up and down check EGR valve and sensor. EGR valve and sensor control idle. That's why your getting the surging on rpm's and this could be giving out the other codes. If mass flow was bad car will not start and the same for the map sensor. O2 sensor measures gas and air for right mixer. TPS controls 0-60 speed as in throttle respons.

A cause for that would be an oxygen sensor getting wet, the one on the inlet side of the catalytic converter.

its vapor locking try leaving the fuel cap a little lose!!!!

Here is the most common cause of surges and stalls and low idle rough idle, it is the idle speed control air-bypass valve and throttle valve (IAC for short), they get full of gunk over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.