jerking at low RPM

Ok I have a 95 Sonoma with a 2.2 five speed manual trans. Since about 90k it has had this little jerking thing it does at low RPM's. It is at its worst at about 25 MPH in second gear.
I have replaced the O2 sensor, the Throttle posistion sensorand the fuel pump. A mechanic told me to try all of these. Now the mechancs says it is the idle switch. I have check the engine and under the dash and can not find anything that looks like a idle switch.
The truck now has 142k and it seems to finaly be getting a bit worse and the gas mileage seems to be dropping a bit.
I think this is probably one of the best vehicles I have ever owned. I have driven this truck from California to Montana non stop,(except for gas) wouldn't even think of getting rid of it.

Thanks

Jon