frequent engine stall during braking

Just recently my 86 300e has stalled now and then when I brake. My idle rpm is around 750. While coasting say at 20 mph in drive the engine idle is approx. 1200 rpm. Generally while braking the rpm drops to just bellow 1000 but now and then the rpm drop to about 400 rpm and unless I catch it, the engine will stall. Any troubleshooting ideas are greatly appreciated.