Runs ok cold. Dogs down when warm.

I just bought a 91 RMW and am trying to figure out a problem.  It starts fine and runs and shifts fine until it warms up. At that point, it stumbles when you press on the gas and has no power. 0 to 60 in 5.9 minutes. Any ideas?

Do you know any service history? How are the common tune up parts such as fuel filter, air filter, plugs, wires, cap, rotor, etc.?

what's the exhaust smell like? rotten eggs?

Is it a sudden transition, or a gradual fade? A sudden transition might point towards the moment the ECU goes into closed loop mode from open loop, or an electronic component heating up.

The car starts in open loop mode, relying on temperature, pressure, etc. sensors to set the mixture. Once the O2 sensor gets up to operating temperature, the ECU switches to closed loop, relying primarily on the O2 sensor feedback to set the mixture. Electronic components such as the EST/ESC system still rely on ECT, MAP, TPS, etc. to control the timing, of course. Usually an electronic component fault will set a code and the MIL (aka "check engine light") will come on (although the ECU can be fooled by wrong data that is still within a valid range). Is your MIL on? If not, does it work in bulb check when you first turn the key? If it is on, what codes are set"?

Like buickwagon said, it sounds like it is going into closed loop, and something is amiss. Check your codes by shorting pins A and B on the ADL connector under the dash, with the car in "key on engine off" mode. The CEL will blink to give you the two digit codes (three times each). A one and then a two (12) lets you know the system is functioning. After that, it will go through any other codes. Write them down, and refer to the FSM for the interpretations.

If there are no codes, check the ground at the thermostat when the engine is warm. Remove the stud, and clean it, as well as the threaded hole. It is the ground for the O2 sensor, and the ECU.

The cat should be fine, because the car runs fine when cold. You can probably eliminate the TPS, PVC, IAC, EGR valve, ignition system, and fuel system. My first guess is the MAP sensor or the O2 sensor. After that I would go to the Temp sensor, and then EGR solenoid.

The computer uses internal references for some sensor inputs when it is in open loop. When it goes into closed loop, it uses the sensors directly. Some affect the performance more dramatically than others. Since it works in open loop, and acts like it is not getting fuel in closed loop, the computer appears to believe there is either too much air or not enough gas. The MAP sensor tells the computer how much air it is getting, the O2 sensor tells it if the mixture is correct, and the EGR can allow extra "air" into the mixture. The IAC only affects idle, The IAT tells the computer what the ambient air temp of the incoming air is (minor issue), If the temp sensor is not functioning properly the engine may think it is overheating, and limit the power available.

Hi Guys, sorry for the slow reply. Mom passed and yesterday I was taking care of affairs. The boys drove it down for the funeral. And since I'm staying a few extra days, I sent them home in the OCC.
It is a sudden transition, so I think that it must be a sensor getting warm

I am going to check on it in a bit and let you know. Thanks for all the suggestions. I appreciate it.

So sorry for your loss,no need to apologize.

indeed, you've got other things to tend to

Thanks guys.  I'm back home now and have time to update you on what was wrong with the RMW.  The old saying that "Timing is everything." is so true.  It seemed like everything we did helped a little bit, but nothing made her get up and run.  So I talked to my uncle and he said that he would bring over his timing light and we would check it out.  Turns out that it was 10 degrees off tdc.  We corrected it and she took off like a horse on race day.    I'll get some pictures of her up soon.

The distributor may have been loose.

Did you disconnect the EST signal (tan/black) wire when setting the base ignition timing? Was there a code 42 set before setting the timing? (if you disconnected the wire to set the timing properly, code 42 will be set now).

If the distributor was not loose, you may be compensating for an EST problem. You will get better performance and economy if the EST system is working properly.