problem with my mixer shower?

i have installed a cascade jive surface mounted thermostatic mixer shower and have a problem with the water flow
if i run it on the top hot setting the water runs fine if i run it on the lowest cold setting the water seems to pulse i.e it runs fast then drops instantaneously and does so on every setting until set on fully hot again
i thought i had an air lock in the hot water tank and so connected the cold supply up to the hot water to blast it back through but this hasn't worked
i have tried the fitting the flow limiters in both hot and cold together and separately and have now run out of idea's
if anyone out there knows what the problem is it would be most appreciated