Shower running hot and cold

This may be a fairly simple issue but I'm paranoid about damaging the pulsacoil!

My shower (which runs hot water direct from the Pulsacoil 2000) has it's own temperature control. The shower runs hot for about 30 seconds then slowly goes cold then warm again.

All hot taps are fine.

Does this sound like a Pulsacoil issue or just a problem with the shower thermostat? Would I need a specialist Gledhill repairer to look at this or could I ask any plumber? Also, if I do get a plumber who is not familiar with the Pulsacoil is there anything he should be aware of when turning off the water to replace any components in the shower itself?

Thanks!

Obvious oplace to look is the Mixer shower valve in the bathroom but if this is all Ok then worth checking :

1) The Pump (which could be coming on and off intermittently) and warming the water up but then stops running and the heat exchanger is not able to transfer any heat within the heat exchanger. To check this - ask someone to run the shower while you keep an eye (or feel) the pump. If the hot water starts to go cold exactly at the same time as the pump stalls then this is a good indicator that the pump is stalling.

2) The Control board,

It may also be worth checking that all mains connections are tight (with all necessary precautions of course).

I would strongly recommend you have the shower cartridge replaced first. This could also be related to the flow rate of your shower, the pump, PSC board or sensors.