1995 Accord Missing

I could really use some help with this one. I have a 95 accord manual transmission and it has the V tech 4 cyl engine. The engine has started to miss in the lower rpm's. Once the engine starts to miss or lug down, I am able to hit the accelerator and it goes back to normal. At high speeds this miss or lug is not as noticable. Here is what I have done so far:

1. Replaced spark plugs
2. Replaced fuel filter
3. Replaced plug wires
4. Replaced distributor cap and rotor

what else could possibly be the problem.

ps. I don't know if it has anything to do with it, but I am in need of a new muffler, I don't know if that could have casued an excess build-up of carbon.

Thank you all for your help

I forgot to add that it only misses under load (usually under 2500rpm)
The engine idles and revs normally when in park.
I also forgot to add that when I pulled the spark plugs they were very lean (stark white)
Thanks