Lose Heat after 30 minute drive in 2000 Malibu, Not the resistor

I lose all air after about 30 minutes of driving. The fan continues to work on all settings. I already replaced the fan speed resistor last year. When driving on the highway for about 30 minutes I will gradually lose all air output. Regardless of the setting, the fan is blowing on high but little to no air is coming out. The fresh air/recirculate works as does the temp control. Then when I stop the air comes back. I am assuming a vacuum leak but I could not find one nor could the local shop. I am stumped. I tried bypassing the selector switch with some tubing and still had the problem. This problem occurs most of the time on the highway, as I stated above. I check everyforum I could find on the web about the N body. I found two old threads on other forums with a similar problem but they had no replies.

Thanks

I have never heard of this, but it sounds as if the intake plenum or a vent line is blocked by something like paper or leaves. The highway speeds making it worse seems like something is being pressed over the inlet. I have not yet taken any of my vent system apart, but I would start there.

Thanks for the reply. I never thought of that. I will check it out and report back. It may take a day or too, it is real cold and windy in NY today.