I have a Baxi 3 solo 50 boiler downstairs and a Gledhill Integrated Thermal Storage -BoilerMate 2000 upstairs. The water pulses every 10 seconds from cold to hot. Very annoying and impossible to have a shower. Called out a plumber who changed the pumps... the circuit board blew up! (actually burnt a whole through it). So changed the circuit board and £700 later OK for a while. Now same again... any ideas?