2004 Pontiac Montana: original problem was heat would go to luke warm when idiling, and fine while driving. it would be low on coolant. i'd fill it up and would be fine.

took it in to find out why it was going low on coolant, and ended up being intake manifold gasket. they replace that along with thermostat and heater core. got it back and no heat to front of van at all, fine in the rear. 7-10 days later after no heat to front of van, now every once in awhile the front heat will be fine, then on the way home from running errands, again back to barely even warm air blowing. it seems like the air is warmer on lower fan speeds when it does work, and turning the fan up higher turns it to colder air blowing out. the couple of times the front heat did work, it was like the original problem of low heat when idiling at a red light and fine when you are driving. i need diagnostic help to rely to the shop that is literary leaving me in the cold help so i can get my wife heat taking our kids to and from school