2003 Pontiac Montana Transmission and heater acting up at t

There is a switch on top the transmission that senses what gear or position the shifter is in(transmission position selector switch). It is located directly on the shaft that switches the gear selection of the transmission. This may be the problem but first if there is a check engine light or service light displayed when this happens, then you should have the vehicle 'codes' read by taking the car to a dealer or quality repair shop to have the computer scanned for codes.
It could also have some shifter linkage/cable out of adjustment, causing it to slip into neutral. Next time it happens try pulling down on the shifter just very slightly to make sure it is still in 'drive', if it feels like it is going into neutral have the shifter adjustment checked.

When I removed the pan and drained the transmission fluid and removed the filter, I noticed a hose (one of 3 or 4 running next to each other) that had com loose.
I reaconnected the hose, replaced the filter and refilled with new fluid.
So far the car seems to have a new problem. From a stopped position the car has made slight quick jumping motion when I press on the gas.
Could this still be the transmission position selector switch?

Does it do this when your in neutral or park? If so it is normal, or I should say "common" for these model gm vehicles to lunge forward slightly when revving up while in neutral or park. It should not move even an inch and it should not jerk, but its not a concern unless it is drastic enough to make the car roll or jerk.
It could be possible that the position sensor on the shift lever would cause a problem such as jerking or with shifting properly from 1st to 2nd etc.

From a stop (car has been in drive and remains in drive) I press the accelerator. The car begins to move forward then, sometimes, it sounds like something slips into place and the car jerks slightly forward and we're moving. After that the car will move through all the gears perfectly. Accelerating, decelerating, whatever traffic needs us to do the car responds perfectly. It's only when we come to the next complete stop that we "might" have the problem again when we accelerate. Any ideas?

You mentioned the heater not heating normally, have you noticed any foaming, bubbles or high fluid level in the transmission? Or a red oily residue in the radiator coolant? If so you may have a leak in the transmission cooler. There are two metal lines that run from the transmission to a portion of the radiator that is the transmission cooler. It sounds very likely that water may be getting into the tranmission and diluting the fluid, causing your problems. I would inspect radiator and trany fluids very carefully for cross contamination. If necessary drain a portion of both(shortly after driving the van so there is agitation of fluids and water/oil will be mixed if both are present. You may be able to get a sample of trany fluid out of the cooler itself by loosening bottom line. A 1/2 cup to a pint of each should be sufficiant enough amount to test) and let them settle for a while, and see if the fluid/coolant seperate, the water should settle to the bottom, trany fluid to top if both are present. I will try to think of anything else it might be if this is not the problem, so keep in touch.

A couple of weeks ago I checked the transmission fluid and it was too high. I syphoned out the excess but I can't be sure if any water came out.
I also noticed there was no antifreez in the over flow the other day when I was working on the car but attributed that to the cold weather and not having driven it for four days.
I'm going to check your idea when my wife gets home with the car. Thank you very much!

Just in case you didn't know, there is a drain valve on the right bottom corner of the radiator(facing the front of van). If you turn it counter-clockwise you can get a sample of coolant here, you need a leather glove or something to protect your hand from the hot fluid.

After Feb 6th repair, when I removed the transaxle pan and while replacing the filter discovered the disconnected hose (which I reconnected) installed the new filter and closed back up.
It's now Feb 24th and the van has been running fine.
I have no idea what was being carried in the hose but since its been reconnected and had some time to get the air out the problem has gone.
The heater is still slow to warm up so I purchased a thermostate and will wait for a free day to put it on.

As a side note. The Haynes repair manual for "General Motors Chev. Venture, Oldsmobile Silhouette, Pontiac Trans Sport & Montana 1997-2005 All models" identified the part "Vacuum modulator" (pg.7-5) that does not seem to be included on the Montana. I spoke to two local repair shops and they confirmed that it's not part of the Montana. Maybe I'm wrong but it appears Haynes made a mistake.
PS Please send a physical address for me to make my donation. I appreciated your help but I don't feel comfortable sending money via the internet. Thanks.