BMW Repair: idle problems, mass airflow meter, radiator hoses

Question
I have a 99 bmw 328i, 56000 miles.  I had the alternator belt/pulleys, and tubes that go to the radiator repaired ($950).  When I got the car back, it was getting very low gas mileage.  So I brought it back and they repaired a thermostat that was dumping gas into the engine($28 for the part + $350 labor).  But it is still burning through gas quickly and when I'm idling, it runs very low, then very high in a matter of about 10 seconds.
What else could be wrong?  and....Should I challenge the dealer that I shouldn't have had to pay for the thermostat since that didn't fix the problem?

Answer
Mr. X  What are 'tubes' that go to the radiator? (Upper and Lower Radiator hoses I'm assuming?). What thermostat was replaced? The coolant thermostat? On what basis was it replaced? What there a fault stored for it? If the coolant thermostat was diagnosed and replaced incorrectly for your complaint then any diagnosis that led to that component should be refunded. They need to explain why the thermostat was determined faulty. If there was a fault for it then it would have caused a check engine light down the road. I would check the throttle boot at the idle valve branching point. Also have them check the mass airflow meter reading. I would calmly request to speak to the service manager and ask if they can help you out with the parts and labor on the future repair.
-Evan