I had the exact same problem on my 2001 Sundance Capri. I changed out he flow switch first but that didn't solve the problem. After confirming the filter was clean, I checked to ensure that the pump was receiving constant power (it was) and then changed the circulator pump. I was hesitant to change the pump because I had replaced it just a few years ago. The prior time I changed it because the bearings were clearly failing and it was easy to determine that it was bad. Thanks for the advice on this thread!