1998 GMC Sierra what is going on here HELP!

Check your electrical connections on the mainifold. Then replace your fuel filter if you havn't so far and get some choke cleaner and clean both sides of the throttle plate as they tend to gum up with oil deposits. If that doesn't cure it have a pro check for cracked fuel lines under the manifold.

If there is a lot of gunk and crap around the tps it could cause it, but you can clean that with choke cleaner and most of the tps switches are covered anyhow. YYou can try it won't hurt. Usually it will set a engine code though. You might check the resistance on the crank and cam sensors and see if it is high. They can go bad and not set a code. The olny other thing I can think of would be low fuel pressure. Yours should be anywhere form 60-66 p.S.I. But I kind of doubt that due to you saying it runs fine after the initial startup.

The O2 sensor could cause the problem of poor idling but it would effect everything while driving. And it depends which O2 sensor it is. There are 4 pre and post- 2 before the converter and 2 after. The post ones don't effect things to much. But if it says like bank 1 sensor 1 that is the one on the left siide before the converter. You could try tit but they are exspensive but they will need replacing anyhow. This could be a coincidence as well. If that doesn't clear it up i'd have apro in your area look at it as intermittent problems like that are hard to find as well as diagnose, especially on here.