1993 Mitsubishi 3000GT Low power.

In the middle of a 30 mile trip, my car started to "cough" as though it was losing spark or something similar. But the end of the trip, it wasn't a cough anymore. It was just extremely low on power, consistently. It takes twice as long to rev up the RPMs and engaging the clutch sucks most of the RPMs, even revved high.
I have replaced the fuel filter thinking it was clogging the fuel intake, but nothing changed. I assume it isn't the fuel pump because the bogging down is constant, never gaining/losing more power.
I replaced all the spark plugs and wires thinking maybe that was the case, but nothing changed.
A friend of mine says that the engine sounds as though it isn't firing on all the cylinders, but I don't know how to go about checking that.
It seems as though it is sucking a lot more fuel that usual as well.
I recently had the clutch replaced, but it ran like a dream for about 30 miles before the problem started.

Any information would be greatly appreciated.