Why would my shower suddenly start running hot/cold/hot/cold etc?

[sleuthey]

Remove the shower head and test the temp consistency at the end of the hose - still the same issue?

 

[Dave7]

So....I cleaned the old head and put it back on. Everything was fine ie it ran hot all the time (no cold spots).
I can only assume the "new head" is too small and causing the problem

 

[Electric_Andy]

That does make sense when I thnk about my old hot water system (and old boiler). You'd only get consistent hot water through if the tap was full on. This was later pointed out to me that it was some sort of valve going/sticking in the boiler. If demand was low-medium then it often wouldn't run hot at all. Maybe the new shower head is not putting enough demand to register for constant "on" hot water? so it's going on and off. When I got a new boiler (bigger and better) the problem completely stopped