Your drain pipe, that drains the condensation water from your ac/heater core might be blocked. (Happened on my 2006). Cost me 50$ at the stealership.
Reduced engine power, on your 2005 Pontiac Montana, can because by several things. The most common cause of reduced power is a lack of fuel.
The most likely problem is a Head Gasket. Sorry Man.
Low Coolant. Poor coolant circulation. Intake gasket. Head gasket.
You are OVERHEATING. You most likely have an intake or Head gasket problem.
Could be low coolant, stuck thermostat, bad heater control head (if climate control)
Good Chance this is caused by a faulty O2 Sensor. I had the engine light stuck on in my 2002 Montana and replaced rear O2 sensor and it turned it off.
It needs to be Re-Set.. Most places can do it for no cost. takes less than 10 seconds.
could be low coolant due to a leak, clogged rad, inoperative cooling fan, thermostats.... overheating conditions can have alot of causes. you need to do some digging.
It could be the electrical ignition system. there is a recall, dealers will replace the ignition relay free of charge. check it out. Pontiac tele. #1-800-620-7668
it could be a range of things. it could be overheating, gas gauge is low, or your rpms are running too high.
Look for the tube that takes the condensed water out of the cabin these sometimes get knocked loose by the feet of the passenger's
What causes a 1998 Pontiac transport to run hot and engine shuts off