89 Cutlass Supreme blower fan

Hi,

I have an 89 Olds Cutlass Supreme. The blower fan started acting weird recently.
Here are the symptoms:using the A/c, when at idle the blower fan blows correctly at any of the 5 speed settings, but at high speed the fan does speed drops to almost nothing and you can hardly feel any air moving. If I let off the accelerator the air flow/fan speed returns.

What could be the problem?

Chris