Intermitent car stalling
 

Looking for help on a 91 300se. It has recently stalled when sitting at stop lights. It begins running rough, then upon excelleration it begins to chug and stall out. This has happened twice in the last 2 weeks so it is not a regular occuance.

How do I go about determining the problem. I have checked vacumm lines and all seem okay. I have left the car run until hot and tried to get it to stall, but it would not. I know it may be the fuel pump, or electrical but how can I be sure before replacing parts.