Triton Power shower, looses power?

My Triton Thermostatic Powershower starts off fine and then after approx 10 mins the motor sounds like it is struggling to pump the water around and the water finally cuts out. If turned off and left it is fine to use again a couple of hours later but the same problem re-occurs. We have taken the cover off and cleaned out some soot like substence. It looks to be an electrical problem, possible overheating??? Any help would be much apprecited. Thank you