Engine sometimes misses or runs rough

This vehicle has been running great for over fifteen years. The check engine light has been on most of the last couple of years. A week ago drove about one hour in mild weather. It ran fine. I shut it off for about half an hour. I started it up (started fine, always does ). Immediately it started idling rough and ran rough for about half an hour, sort of like it was missing. The check engine light was flashing the entire time. Since then, it has been running fine (short trips, cooler weather). The check engine light
has been off. I took it to a repair shop. They said the computer did not show any codes, so they could not do anything (and did not ).
Any ideas?