AC stops working under long drive?

[Dus]

Hi fellas

I just got home from going to New Orleans from Houston.  I've noticed that
after 3 hrs of straight driving going to New Orleans, the AC seems to stop
working properly.  The air is still pretty cool (not cold).  You can hear
the fans running at max speed when i set the fan switch to "4", but there's
just a tiny bit of air coming out of the vents.

After stopping for gas, the fans start blowing strong cold air out again
like normal.  On my way back home, it happened again.  Fans are working at
full speed, but only a little bit of air is coming out after 3 hrs+ of
straight highway driving.

Anyone have this problem?  i searched the archive but i don't see anyone
experienced this yet.

Also, i realize there's a lot of moisture under this black box behind the
glove box.  water drips out of it when the AC gets hotter.

Thanks.
Dus
was driving my girlfriend's 98 sentra SE auto.