air not blowing through vents properly

i have a 99 grand prix and have had an issue off and on through the past couple of years with the air vents. when driving around town at lower speeds, the a/c works perfectly. however, when i travel on the interstate, the air quits blowing through the vents after probably 30 minutes or so. everything sounds normal except that air is just barely blowing through the vents. i've tried changing different settings but to no avail. the air that does come through the vent is still cool so i know the compressor is still working.

however, if i stop or slow down for several minutes and turn the air off, it starts working normally when i turn it back on.

last summer i did have the entire a/c system replaced because of the compressor bearing. after some leak issues, i think it is finally repaired. i mention this because during that time, i didn't have any issues with the stoppage in air flow. now it's back and it's hot so i want to get this fixed once and for all. any ideas? i'm suspicious of a leak vacuum hose but that's just a guess.

There is a hard plastic vacuum line that feeds vacuum to the HVAC unit to control mode switching (heat to vent to defrost, etc...) that gets britle with age and is prone to cracking. It runs from the intake plenum, along the fenderwell near the battery, then back into the passenger compartment. It's also fairly common to break it if you did any work around the battery. If you find a crack or break, you can repair it by cutting out just the cracked section and splicing it back together with a piece of rubber vacuum hose that fits snugly over it rather than replacing the entire plastic line.