wait a second..........did you say you have a coolant leak?????? from where exactly? if you dont know where the leak is coming from, check around the #4 spark plug. (#4 is the one closest to the firewall, on the passanger side). might be that there is coolant in one of the plug holes, and while its cold, it causes a misfire, once it warms up, the coolant goes bye-bye and there is no more misfire. the ECU wouldnt pick up that small of a misfire.
just an idea though.