Slow running engine problem

What is the cause/possible remedies for toyota Avanza 2008 model with slow running engine problem. This happens anytime of the day be it early in the morning when the car is just been turned on or late in the day when in traffic. The problem normally cause the car engine to stop at any slight removal of leg on turtle. In some occasions, the slow running becomes high running problem as the engine run so high that the speedometer get to 4 while not on motion. At this point, I will be forced to turn off the engine for fear of car getting burnt. Some mechanics said it could be that the turtle cable is damaged but on a check the cable is very much OK while another mechanic said we need to service the turtle mouth. We are tired of the trial and error solutions, kindly help