buzzing then inconsistant hot water

Hi, We have a BMII which is around 14 years old. For over a year we have had a buzzing noise about once a day for around 3 seconds. Upto now we have ignored the buzzing as it's only really noticed if it happens at night. More recently the water from the hot taps runs hot and then quickly goes cold. This change happened litteraly overnight. The central heating seems to be working fine and the water store is hot. Since investigating and looking at previous posts I've found that the buzzing is coming from the delay timer pcb and also from the pump pcb. I've order 2 new pcbs and waiting delivery. I've also noticed that the pump from the boiler is running consistantly. Is a possible that the limited hot water could also be caused by the PCB and continuosly running pump problem or is it likely to be the mixer valve ? I've turned the mixer valve and it seems to move freely and when closed stops all hot water. Thanks for any advise you can offer.
Andy

I've now replaced both the PCB's and this has cured the buzzing and the pump continuous running problem, however we still have very little hot water so I expect the valve does need changing as well. Before running the hot tap both sides of the valve and even the cold intake pipe are hot but once the hot tap has been running for a few seconds the cold intake pipe and the output pipe from the valve go cold. My concern is how easy this valve will be to remove as it's been on there for 14 years now.
Andy

Replaced the valve and now have hot water on tap. The old valve come out easily having turned off the main water supply and turned on upstairs tap to drain pipe. The plastics inside the old valve were crumbling when touched.
Andy