'00 XL slow to crank/idles rough while stopped

[ahfann]

I have a '00 XL that cranks fine in the morining, but is slow to crank afterwards. I have 35,550 miles on it. It has had the charging system tested-OK, fuel pressure-OK, injectors serviced and cleaned by GMC, fuel pump and filter replaced by GMC, an upgrade to the prom chip for stall and rough idle by GMC, new battery by myself. I still however have trouble with it being slow to crank at times, and it has never been duplicated when at the Dealership. When it finally does crank there is an odor from the exhaust. Can anyone please help me with this problem. Thanks in advance for your help.