1987 Reliant air from vents goes away when accelerator press

1987 Plymouth Reliant, 50000 miles, V4

The air blowing out from vents. On Panel, air blows out strong until pressing accelerator pedal where air goes away and suddenly starts blowing out floor vents. On Bi-Level, air blows out both dash and floor until accelerating, then all air comes out floor vents only. On Floor, works fine. Air blows out dash vents on Panel setting only when not accelerating or car idle. Do I have a vaccuum or suction problem with a hose that's rotted? Is this something I can look for and fix? It does this whether on AC or fan only. AC does cool but will only exit floor vents when accelerating, no matter what position vents are set on. Any thoughts I would appreciate.
Extra information: Back in 1988, this car had dye put in AC system to find a leak. Freon has leaked out in past, but has not needed recharging in 2 years and does cool.