hi i have a baxi bahama 100 that has been nothing but problems for years.the current problem is that 2 rads down stairs and 2 up stairs are very hot and the other 4 upstairs and 4 down stairs are cold or luke warm.so far i have bleed and flushed the system,manually checked diverter valve all ok.the only thing that appears strange is that when the boiler starts up to heat ch it appears to only stay on for a few seconds i haven't really checked the pump.what would be the best way to check for this(the pump is grundfoss type up 15-60 up).oh also the hot water seems to fluctuate in temp.any suggestions would be appreciated thanks