x10000..
Ive ran into this numerous times at toyota.. If there is no problem when its in the shop (which happens so many times) There isnt anything you can do.. BUT if its a common issue, I replace the part under warranty and go from there..
I agree with cordell.. Im flat rate... If I cant get it to act up within 30-45 min, im not working on it anymore... I diagnose it as no problem found.
If it were to occur in shop, there are multiple things we can check while its in the shop...But if it works great everytime its in the shop, its frustrating, but nothing can be done..
And you dont need to keep records of EVERYTHING like Ziggy said.. Anytime it is in the shop, there is a RO open, and you get a print off of what your vehicle is in for.. Anytime you have an issue after your warranty is up, the service writer will look up history and know its covered because of previous issues..
I know how frustrated you are.. But the only thing you can do like others have said is hand over the keys and let the mechanic drive it... I wouldnt want to do that with my jeep.. but thats your only option really..
And the mechanic needs to drive it, not manager or service writer... They typically dont know much about diagnosing.. just what the mechanics tell them..