we had the same issue but it was a limescale issue in the head - only hot water was getting out. Got one of the liquid descalers, left shower head in for 10 mins - perfect, back to normal.
we have the same prob and every now and then take off the shower head and put in descaler. Some times if you are in the shower when it happens let the shower head hang down and it seems to work ok again, dont know why
sounds like a partial blockage somewhere in it... suggest you get one of the shower maintenance guys in to have look at it. Problem is most of these showers only seem to last 6-8 years at most