In my car (98 gtp), and my parents car (95 buick regal) the vacuum line going to the climate control was cracked. I replaced mine with a piece of metal brake line along side the transmission selector cable, shortening the line significantly.

The crack in your line must be getting worse because I am running without a vacuum reservoir to my knowledge and the mode does not change on me, even on heavy acceleration. You should fix the line as it is a unmetered vacuum leak with unfiltered air.