Hi,

I have a '98 GMC Safari.

My top blower vent seems to have some kind of problem. It blows very lightly occasionally and full power at others. The fan sounds normal -- it comes on and you can feel it blowing through the lower vents consistently.

The best I can try to do to replicate the problem consistently is as follows:
-set the fan on high, blowing through the top and bottom vents
-accelerate very hard from a dead stop
-by the time I'm at about 35-40 mph, the fan starts blowing weak
-when I hit the brakes and start slowing down again, the fan comes back on full power

The top vent comes on and off while driving on the highway and around town.

I recently had the shop reconnect the top vent, which before was not blowing at all. They said the hose that connects to the vents was disconnected. I think they must have reconnected something improperly.

Any ideas?

Thanks,
Steve
'98 GMC Safari

Here's my one and only idea. I had a similar problem.
I could hear the blower working but very little air was coming out the vents.
The A/C controls are both electrical and vacuum hoses. In my case the problem was resolved by finding a loose vacuum hose in the A/C system.
Check them for holes and split ends as well.