I had this same problem however, it seems that i only had 2 of them firing.
so, I replaced my spark plugs and no fix. Then i replaced my distributor cap and rotor and no fix. then ignition wires and no fix.
then i removed my injectors and bench tested them, They seemed fine however, I took them into a shop and had them sonicly cleaned. Put them back in and it fixed the problem.
I also replaced a bunch of fuel line during that time.
Car is running great now