Worcester Heatslave 15/19 - water runs cold every now and again?

I have a 10 year old oil fired heatslave. The heating is good and usually the water is too but now and again it will run cold when running a bath. I thought it may have something to do with the speed that the water runs out of the bathroom tap and running it slower does tend to make the water hotter, however, on occassion it will run cold no matter what speed I run the tap. I usually turn it off and let it be for a bit and it usually then comes back hot again but not always. It really is hit and miss if it's going to run cold. I have had issues with the bathroom tap and water pressure and hammer so I don't know if this has any bearing on the issue, the kitchen and bathroom sink hot water is fine but they are not on as long as a bath tap is so I can not say for sure that it is the bathroom taps as I have nothing to compair them against

has anyone got any ideas?

PS - we have had issues with leaks too in the boiler which are due to the joins becoming loose and they just need tightening to fix the leak - not sure if this is tied into the issue of the water going cold as maybe our cold water is too high pressured?

thanks in advance for any advice you can offer