Engine warning light comes on intermittently but car seems to be running normally. on 2001 Volvo V70 XC

My '01 V-70 XC occasionally but not always has a "Service Engine-Urgent" and orange ! light come on when I start the car. Operation is normal and engine seems smooth so I plan to drive to the dealer tomorrow for a diagnostic. Last night the engine was rough and wouldn't smooth out--couldn't get going over 20 mph so took it right home. Today, no problem and no warning light. Any ideas?

Asked by for the 2001 Volvo V70 XC
Sounds like you have a misfire condition that is intermittent. If the engine light ever blinks, then you have a catalytic converter damaging misfire and you should drive your car as little as possible until the problem is completely repaired.
Thanks dandd. so far no blinking so I've got that going for me. any ideas on what would cause the intermittent misfire? bad plug or something similarly cheap or probably something worse?