my truck shuts off while driving and kicks on me. why?

[JustinMathias]

i bought this truck and it idled really high(up to 30 MPH without touching the gas) so i took it to goodyear and they told me the EGR sensor and the ICA motor were bad and i didnt have a thermostat, so i replaced all that myself and now it idles fine and isnt too high, but now its like it doesnt know how much to idle at or something......when im stepping on the brake or just sitting their the RPM's bounce up and down, and it'll go all the way down to 0 and shut off on me. or it'll just real high to about 3 and kick me forward for a second. i thought maybe it was a faulty part or something so i took the EGR sensor back and the ICA motor and got new ones and put those in, but its still doing it. and its getting really bad to where i cant go anywhere without it cutting off and if i sit still usually i need to keep my foot on the brake and the gas to keep the RPM's up. so i took it back to goodyear and they called me back and said they didnt have the diagnostic machinery to figure out what was wrong with my truck and told me to come pick it back up. they told me to take it to a dealership which i know will cost me alot of money. does anyone know what might be wrong with my truck or what i could do to maybe fix it?