i
have a 93 riviera which has 57k on it... when starting it for the first
30 seconds after started it runs rough ,then it runs fine after that..
ive checked the plugs and wires and air filter and o2 sensor but it
still does it and it also doesnt get great gas milage</span></font>

when you replaced the plugs, were they very black?

one bay seemed to have run richer then the other. the bay closest to the firewall seemed clean and the other one darker

any check engine light on? has it ever overheated badly?

never had a check engine light on or overheated badly. could it be a bad egr valve

Try an air intake cleaner and a clean of the vacuum system. Many garage
do this for less than 50$. Your car does'nt have much km i don't think
it could be the EGR valve.