If one mechanic “fixed the problem … only briefly” then go back and ask him what it was he did! Frankly, I think MPG at the higher numbers is going to be variable in any case. This engine is most likely a common rail and so pump timing is not an issue. If it is a PD engine than there is no pump on the timing belt. So, if the cam timing is correct there is nothing else to check. FYI, my '03 TDI auto never returned better than 33 from new, although the DSG should be better.