check engine light\ over heating

y 1999 crown victoria started to overheat while i was sitting in a parking lot with
the engine running.  I notice the a problem with the engine as it began to shut
down and the check engine light came on. Prior to this i had turned on the
defroster to clear up the window. thinking that the thermostat may be the culprit i
had it replaced. i then had the check engine light reset (original reading said that
cyclinder number 5 was at fault). that may have been true since the service station
didn't put on the correct rubber ring for the thermostat and coolant may have
leaked into the cylinder. Since this time i have corrected the coolant leak and still
the check engine light comes on around 55 mph and begins to blink then stays on
steady. do i have to replace the spark plug in cylinder 5? or do i have a more
seripus problem.