It sounds like the TCC is not engaging(in effect, No OD) so this would be one reason(there are possibly others) why your Dad'* PA is getting such bad gas mileage. I am not the transmission expert here(others will be on board later to help) but if the TCC is not locking up properly, it may cause the xmission to heat up and fail prematurely. So the first thing I would do is check the condition/level of the transmision fluid. That is, what color is the fluid? Red, Brown, Black???!!! You also mentioned that this problem has been going on "for awhile". This is NOT a good thing, avoiding what may have been a simple fix that may turn out to be a major expense to remedy.
2006 Salsa Vibe
2002 Snap Orange Beetle - Turbo
1991 Olds "Orphaned" 88 - Donated to Science!