Intermittent heat?

Sometimes heated air is blown, sometimes just cold air. Restarting the engine sometimes corrects the problem for that trip. After the engine turned off, lots of gurgling sounds behind the speedometer. This has worked fine for several years; the problem is only recently. When heat does come out, left and right-side temperature control sliders work okay. I have read of a valve that turns hot water off during A/C use but can't find it. Maybe vacuum activated? (The car had the A/C compressor removed before I owned it.)