1998 heating problems

I have a 1998 pontiac trans sport montana. When I turn the temp. dial to the hot position I will lose all fan speed. The fan speeds all work 1-5 and they will blow just fine when the temp dial is in the cold position. It is when I turn dial over to hot that I can hear the fan still blowing but with my hand to the vents, I can feel 90% of the fan speed leave.

I had read some posts that sound like they have the same problem but I have not read anything as far as a fix is concerned.
Has anyone else had or heard of this problem fixed?
Any info regarding this issue would be appreciated.
Thanks

You may have a malfunctioning damper or plugged heater core. Kind of an odd problem though.