I believe the A/C will cut out if there is a highish load on the engine, or the alternator is not quite putting out enough charge, usually at low RPMs (matching your problem).
The heated rear window will do the same - while stationary, try switching it on. If the light flashes, it has turned itself off because of low battery charge (happened to me when my battery went bad) or bad alternator. Check your battery voltage at idle and post back
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black