What is the usual reason why a 1996 Maytag gas dryer doesn't heat properly. It seems to finally start getting warmer near the end of a long long cycle on Regualr heat, timed drying (or any of the other cycle choices)

I would start by making sure the dryer is free of lint. Not just in the screen, but in the exaust, around the blower, the exaust hose, etc...