no hot water from shower mixer/combi set up

[meckev]

my shower mixer runs off of a combi boiler. but it never runs hot. if you turn the mixer on a little bit the water comes out at like a normal electric shower, at a decent pace, but there is no temperature control, the temp dial does not have any effect. sometimes the water will come hot for a moment but then go icy cold again. if you open the mixer fully, suddenly it runs at a realy high pressure. and stays cold. again the temp dial does not work no matter where it is. is there anything i can do to make this system wwork?

[plumbbob]

Is this a thermostatic shower? Has it ever worked? Was the combi installed after the shower was fitted? What make/model of shower is it?