Chevy Tahoe acting up and can't diagnose the problem.

I just bought this 1999 Chevy Tahoe about one month ago, and since the purchase I ahve had nothing but problems. The fisrt problem I have been having is with the SES light came on during a 45 min road trip to a friends house, no rough idling or anything or that nature, decided to get the SES checked at a local auot parts store, and it came up with a P0420 code, which from what I have researched it is a catalyst efficiancy problem. They told me that me cats needed to be replaced. So I decided to just wait and then three days later the SES light went off. And about a week later it came back on carrying the same code. Checked with the local dealer and they told me the same thing about replacing the cats. But the codes has cleared it self again and hasnt been back on since, but now I am noticing poor throttle response and some time times not very frequent when I go to start the truck it will fast idle then drop out and shut off and it takes a couple of times of this happening before it stays running. Im not sure if the fuel pump, spark plugs, fuel filter or anything of that nature have ben replaced recently but I would like to know what I cant do to get my truck starting with out the hesitation and get my throttle response back aswell.