spends over an extra 5 seconds before cranking?

i have a 2000 chevy suburban when i try to crank it in the morning or on a regular drive it spends over an extra 5 seconds before cranking. sometimes after driving for a while and stop at a light it has a rough idle. i have put it on the diagnostic machine, checked o.k., i have sent it to a mechanic he cleaned the trottle body still contiue to do the same thing. i dont know what else to do i hoping its not the timing going bad.