problem with t90 si shower?

hi lads, help please if u's can...i got a triton t90 si shower, (had it for 5yrs brillent unit),but laterly, if i have shower for 10-15 minutes, then i switch it off n get out. & then the missis gets in switches it on, it can go from set tempurture to cold (with a different working sound), then after few sec's it will go abit hotter than setted temp', then cold again... if i/she switches it (the unit) off for approx' 1 minute then back on again its fine... the unit works fine most of time, only plays up if i or the missis as a shower gets out then i or she gets straight in.. is this a safety thing on the unit or a cool down time on it.. i keep the filter & head clean as we live in hard water area... thanks all for u help...