Mixer shower intermittent problem

Just had a Triton Dove mixer shower installed in conjunction with a Salamander CT75 pump.
Bit of a strange problem. When I start the shower after it hasn't been used for a couple of hours, I get cold water coming through in pulses.
If I remove the shower head from its holder and bring down to bath level, this solves the problem and the shower operates normally. I can then return it to the holder, and shower normally. When I switch the shower off then switch back on again, everything is fine again, the problem only seems to happen from a 'cold start'.
Any ideas?
Many thanks
Alex

where is the pump ?
hows the pump/pipework routed ?
wheres the hot taken from how ?
wheres the cold taken from how?

need more info

The pump, cold water tank and hot water cylinder are in the loft.
Cold water tank sits above the hot water cylinder, pump is mounted on the floor in line with the tanks.
The hot water is taken from a direct feed from the hot water cylinder, then through the pump, then pretty much straight down to the shower.
The cold water is taken from the cold water tank, then through the pump, then straight down to the shower.
Cheers
Alex

Sounds like its possible that the flow switch on the hot side in the pump is not being triggered? When you bring the shower head down further, there is then enough flow pressure to trigger the switch and you get your shower.
Would like that clarified though...

Yes that's possible. When I put the shower head down in the bath, everything kicks in after around 10 seconds.
There are 2 dials on the shower, each side of the bar mixer, flow and temperature.
With the shower in its normal position, I have also found that if I start off with the temperature at cold, and a relatively low flow rate, I can gradually turn up the temperature and flow and the shower will work as normal. If I go too fast on either temperature or flow, the shower starts sputtering again.
I'm pretty sure that this is a pump issue rather than a shower issue?
Cheers
Alex