My 98 olds 88 has been stalling lately. usually

Backfire is a sign you fuel/air mix is wrong. The backfire is the excess fuel burning in the exhaust. Have you got any warning lights on the dash? When was the last time it was last given a major service? A dirty air or fuel filter can cause issues.
Check your Air intake and fuel system. It could be starving the engine of air or supplying too much fuel to the injectors.
It happens at over 55mph because the engine is working hard and fast pulling a lot of fuel through to do it.

First you add octane booster to two year old gas. It is too late for stabil. Find the idle air solenoid and take it out and clean it with throttle body cleaner. Try not to push it in or out. Reinstall and turn on key for I min. and off for 1 min. then start the car.

You either have a broken gas tank fuel pump well baffle or the fuel gauge is incorrect and you really are about to run out of gas. Both requires the fuel tank assembly be pulled out of the car and the baffle checked around the fuel pump or the fuel pump replaced if the baffle is ok.

Did you ever find out what was causing this? I am having the same problem.

My first guess would be the fuel pump. Alot of defective pumps will operate until they warm up.

I would check the connector at the tank and also the grd for the pump

Thats not the most info filled post ever.

I assume you mean you have a car that doesnt start...and you have verified spark, and you pushed the schrader and no gas is in the fuel rail when cranking? ANd maybe even you sprayed some quik start or carb spray into the air filter tube then cranked it and it fired right up and ran a few seconds and stalled after the spray ran out?

If you are REASONABLY confident of all that, GMs of that era are notorious for sudden fuel pump failures. The checks should involve voltage tests, circuit verification, and current ramp testing with a scope before you go ordering a 300 dollar fuel module...
However you COULD take a large hammer...BANG FAIRLY HARD on the gas tank while someone else cranks the engine, see if it starts. If it does, this is almost conclusive proof of a fuel pump problem. DONT DRIVE IT, it may stall again soon or likely fail to restart(Especially if its a VORTEC engine!). If the banging DOESNT make it start, it still COULD be the fuel pump...but it could also be other things that require more testing...and since your info and data is nonexistant...thats all the sound advice I can give you for now without some test data and vehicle and conditions info./

youre best thing is always start with the cheapest route...i mean it can be several of the following things, such as fuel related problems. you may not be getting a consistent amount of fuel to your injectors, or your injectors could be partially clogged....try this first, get a bottle of fuel injection cleaner at any auto parts store..and then dump the whole bottle of it in your gas tank and then fill your tank with gas....this will help mix it up with the fuel..and then see if that helps...only put the entire bottle in because they usually tell you a full bottle has to be mixed with a complete fill up...if that doesnt do it, then it could be your fuel filter, try that first and let me know

I know it sounds crazy but check to see if the floor mat is blocking gas pedal. I have seen this alot in my 30 years as a master tech.the way you describe it sounds like all the other cust of mine that had the same issue. hope that is all it is. good luck and let me know.