My 99 yukon will run fine some days and sometimes

Plugged fuel filter, or faulty fuel pressure regulator. Last possibility would be bad pump but since it does run, it I would first change fuel filter, then regulator if necessary

You might want to take a look at a Haynes manual, as I believe there are two (2) fuel pumps in the Yukons and Suburbans. I had one fail and because of that, it would hard start after some trips and other times it would be fine.

MIght well be worth the small price to buy a repair manual at your local auto parts store, just for the troubleshooting guide and pictures showing the location of all the key parts.

Hope this helps and best regards!

either your fuel pump is going bad or the relay to the fuel pump. Have you changed you fuel filter lately.