recently had a "mechanic" replace the timing belt in my 2010 Jetta TDI Wagon. The MPG dropped from 50 to 42 and it started running a little off. Took it back, they tried again and it went to 36. Took it to another mechanic who said (and provided a bunch of data...) it was installed incorrectly. Took it back, tried it again and it went to 40. Took to another mechanic in a neighboring city, he fixed the problem but only briefly. It has slumped back to 42? And runs a little off. Any ideas? Did these first guys destroy my vehicle?