Coolant Leak. On & Off

This AM while at the drag strip I was pulled off the line as the car was leaking coolant. (steady flow from the back of the engine area.) By the time I got back into the pit area the leak stopped and could not be located. Drove back into town (about 5 miles) and the leak started again. By the time I got the car on a ramp the leak stopped. Topped up the overflow reservoir and everything again seemed OK. Went back to the strip only to have the leak start again. Went back into town and found that it wasn't leaking again. Checked the overflow reservoir and it was down so it was again topped up. Decided to call it a day and drove home (about 120 miles). Temperature gauge was reading normal. Stopped to check coolant level after 10 miles and noticed that the reservoir was a little lower. Topped up again and drove home. When I arrived home the coolant reservoir level was unchanged (full to the top). I have never ever, ever, seen a leak like this (steady stream of liquid) just disappear. Leak was at the rear just in front of the bell housing. Heater hoses were dry. Can anyone help identify the source and why it would start and stop?? It reminded me of a pressure relief valve going off and then reseating itself. Other than the radiator cap is there another similar valve somewhere in the cooling system that could be failing?