1996 Geo Prizm EGR Valve trouble?

My 1996 Prism has no check engine lite showing-I drive it for alittle while-it hesitates increasing to actually "bucking" and has actually stalled. When it seems to have a loss of power-I can pump the gas pedal a few times and make it continue running or it can stall also. We have cleaned the EGR valve/ERG vacuum modulator-it was clogged with thick black carbon gunk-but the car still does the same thing. We've also replaced the spark plugs/ignition wires, diistrib. Cap, oxygen sensor. My car has original engine/transmission and has 321,442 miles-Can I fix it to keep it running? It is a FANTASTIC Car-my best buy ever! Thanks for any help/suggestions! Nancy