Possible timing issue

I have a 99 Tahoe 5.7 with 120k miles on it. Have been having trouble with it starting. Warm starts or cold makes no difference. Some starts the truck will fire up instantly and others will crank for 10 seconds or so before firing and then very labored until idle comes up. Been having problems with it for awhile. Had my fuel pump go bad so replaced that and also my secondary cat. Still starting rough. Backfired acouple of times which makes me lean towards timing belt may have slipped a tooth. Truck has unburnt fuel smell from exhaust and kinda sluggish while running. Just did a complete tune up and little difference. Apparently I need a scanner to check or adjust the timing. maybe O2 sensor going bad? Haven't checked the fuel pressure yet, new pump but not sure about the regulator. Any ideas??

No T belt here, and yes check the FP regulator, may well be the issue here. Step one.

Definately check to see if you are losing fuel pressure or that the fuel pressure is right at the low end of the specification.
If so, the regulator is a good place to start

Got in the truck yesterday and my check engine light came on. Truck ran like normal (very weak) so I ran to autozone to get code read. gave me a bank 2 is insufficient for run conditon. Can't remember the code. Said it was my O2 sensor on passanger side. just replaced the cat on that side, could that O2 sensor be the problem? Seems like since started running the 10% ethenol gas in it, been having problems. have an O2 sensor in my garage i can try.
Thanks guys!

We will need the exact code for any meaningful advice.

Sorry about that. The code was P0430. Changed out the cat. but still running weak. This morning I got a misfire code. P0300 I think. Very random, cleared the code and didn't come back. I have remote start, and I think when the truck started it may have backfired through the intake causing the code. What do you guys think about the CPS? Could that be bad, causing the hard, labored starts and rough idle? Guess I need a scan tool to check huh?
Thanks guys!

Correct and check for the FP in the system holding when the engine is shut off also.