1989 Trooper II Engine Surge Problem...

1989 Isuzu Trooper II 4 X 4
Engine: 4-cylinder, 2.6 liter, 4ZE1 Fuel Injected
Five speed manual transmission
Vehicle Identification Number: JACCH58E5K7909069

1) Engine started surging up and down about 400 to 500 rpm from about 1300 RPM to 1800 RPM when I let off the accelerator at idle. It also idles fast when this is goiing on... What might cause engine surge? What might fix it?

2) How much does the smog control pump help gas mileage, if any? Can they be rebuilt? What does one cost?

Thanks in advance,
Al in Florida Panhandle