Toyota durban north not very helpful

I own a toyota starlet 1.4 been serviced regularly at toyota durban north .recently booked the vehicle in as it started to over rerve occasionally. When the vehicle went in was advised by the service advisor that nothing is picking as they picked up the thermostat house and pipe needs to replaced..then I called them back to advise that over rerving is still on and off in the vehicle. The service manager advised me to bring the vehicle as we wants to drive it himself .took it to him he drove did not over rerve and he advised to take out videos if it does .so after a day it started doing the same so sent the videos called him asking him if he did a diagnostic on the vehicle he says they jus went through and didn't pick anything. Really frustrated as it's costing me alot to run to and from to them