I have a '91 Toyota SR5 4 Cyn. Once engine warms

Should be on or near the fuel injector supply rail.

It can be issue with fuel pressure,its low or the problem relates to faulty spark plugs.
There are few troubleshooting steps to confirm the problem.I suggest you few help links,with complete troubleshooting and solution for these types of issues.

1991 Toyota Trecel: Engine Starts fine but dies when gets Warm? http://whatisbyme.blogspot.in/2012/05/1991-toyota-trecel-engine-starts-fine.html

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1994 Toyota Pickup: Starts fine but dies in few seconds?

http://howtobyme.blogspot.in/2012/05/1994-toyota-pickup-starts-fine-but-dies.html

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1994 Lexus ES300: Engine Idles Rough and Dies? http://whatisbyme.blogspot.in/2012/05/1994-lexus-es300-engine-idles-rough-and.html

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Stalls And Dies At Stop Sign? http://www.howtobyme.blogspot.com/2012/07/stalls-and-dies-at-stop-sign.html

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Car dies when gas pedal is pressed? http://schematicsdiagram.blogspot.com/2011/12/car-dies-when-gas-pedal-is-pressed.html

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These help links will help you to troubleshoot and solve the issue.
Thanks.

My 1990 S10 pick-up had a similar problem too. It would start up fine (while the engine was cold) then once the engine warmed up it would idle very rough, sputter, and even stall out. My grandpa and I found what was wrong finally. The fuel distributor had been cracked, so when the engine warmed up it would miss (giving either too much or not enough fuel). So I recommend that you check that. Hope this helps.

ive had a problem like this before it turned out being a shim under the camshaft worn and when idle it wasn't pushing the valve down properly.all it took was shimming it out.might want to check that out pending your mileage.

HI, i would advise to check the following areas for faults. replace any part that is fouled or malfunctioning.

1. Check the air filter- This device may be clogged or dirty. replace if visibly fouled.
2. Check the fuel pressure regulator- pull the vacuum hose of the regulator. if there is gas in it, replace the fuel pressure regulator.(if there is no fuel present in the lines, move on to the next check point)
3. Replace the fuel filter and, check the voltage at th fuel pump. you should get 12V at the pump. if its lower than twelve, chekc the ground for corrosion as well as the entire wiring harness as well. all the way to the relay for fualts.
4. Check the Intake Manifold Gasket for worn or damged edges. this ware and tare will cuase leakge. replace this gasket if found to be fouled in any way.
5. Check the freash air intake hose. this hose may be clogged or damaged.
6. Last but not least, check the injectors for leaking seals and, correct pulse as well. replace any injector with a weak pulse or , leaky seal.

Change the air temp censer.

check the choke setting

Something is riching or leaning this vehicle out at the wrong time. I'm gonna go with ERG valve being stuck in the open position. The only time this thing should work is at crusing speeds. Once the car has cooled down the fast idel cam takes over and prevent it from stalling. Once temp is within range idle cam stops and EGR is dumping into the mixture.
Use shop manual to do proper testing for the valve( probably a hand held vacuum pump is all you will need and gum out to free it up but you should properly replace it if it turns out to be the culprit.

Mine did the same thing at 16K miles, I cleaned the throttle body and the ploblem went away. Took me about 30 minutes.

Check the fuel system, chenge fuel filter. clean the fuel tank.