No Hot Water?

The hot water on my boiler goes cold if I am running the water continuously for around 5 minutes with the heating on. I was checking what was wrong with the boiler and ran the hot water tap whilst the heating was on. I found that after a few minutes the water pressure appeared to go down. The water went cold at this point. After a short period it would increase and the temperature would go up again. I was checking this for about 15 - 20 Minutes. During this time the radiators went cold. What could be the cause of this problem?