96 Chev C-1500 4.3L "Service Engine Soon" light problem

I have a '96 Chevrolet C-1500 4.3L that has been giving me a bit of a
problem as of late. When I am sitting in traffic at a stoplight, the
"Service Engine Soon" light will come on and start flashing. Then
approximately one minute after I start driving again, it stops
flashing and stays on. Then when I come to the next stoplight, it
starts flashing again. I read through the owner's manual and it says
when the light flashes, it is most commonly due to an engine misfire,
and to take it into a dealer. I'll be taking it in tomorrow to my
mechanic (whom I trust a whole lot more than the local dealership),
but want to get some feedback to see if anyone else has experienced
this or what it could possibly be before I bring it in. In any case,
if it's big money to fix, I'll have to retire this rig and go get a
new one.

Thanks for you help.