Combi Water Pressures & Mixer Shower Problems

On fitting a new Triton thermo mixer shower to the system, I'm having problems with the shower pulsing with pressure and temperature (the temperature drops on the shower and the pressure increases, it does this every 2 to 3 mins and then drops to normal again). The hot side of the system is fitted to a combi at 3 bar at the bathroom, the cold can either be ran from a tank in the loft at 0.5 bar or direct of the main which is strangely showing only 0.25 bar.
1) Could the problems be a result of the pressure differences between both sides even though both sides are within limits of the shower (5 bar)?
2) How can both sides of the mains fed system through the combi have different pressures?
3) Is there a simple fix for this?

Many thanks

 

No this is by a mains pressure gauge put on the sink feeds in the bathroom.

 

There are iso's fitted on both feeds but all are fully open.