What could cause a surging idle and stalling motor in my 1995 Isuzu Trooper?

My trooper has been running fine until yesterday when I tried to crank it. It starts after a bit of trying but the idle jumps between 2000 and 500 rpm. After two or three cycles it sputters and dies. If I try to keep it running with the gas pedal it still surges and eventually dies. About a month ago I replaced the spark plugs and wires, air filter and fuel filter. I have checked fuses for the fuel pump and they are fine. What is my next move?