I have had issues with the throttle pedal potentiometer before and gave symptoms similar to this
You have to 'dial in' the switch which rotates to a given setting when you measure the volts between a certain range....problem solved at little if no cost
Also, check your earth straps from the battery to chassis, chassis to engine and also gearbox as these break and can cause strange faults
Finally look for vacuum leaks which can also cause the engine idle to hunt