Starting issue, please help..

I have a '98 240, all stock.. I've been having problems the last 6 months starting the car either bone cold or not driving it for 4-6 hours.. the car would startup, then bogs to almost stalling, goes back up in rpms, bogs again and does this at least 3 or 4 times before idling normally.. at one point in time when i had my HKS exhaust, the car would just die on start up unless i gave it a little gas after i started it up.. i've recently changed the spark plugs and fuel filter and using my stock exhaust system and the problem does not seem to change.. I'm thinking it could be the fuel pump since it seems like a fuel issue.. let me know what you guys think it is.. thanks