Vokera Excell 80E overheat problem

Hello! Please can someone help me out with this problem. I have a Vokera Excell 80E boiler which works fine most of the time. The problem is that when I have a shower it will overheat and lock out meaning I have no hot water. It eventually resets itself and starts working again

This problem occurs most times that I use the shower. Sometimes I can be in the shower for 10 minutes and it will be fine, other times it will overheat after a few minutes.

It happens regardless of whether the central heating is on or off at the time. The amount of time it takes to overheat varies but I'm sure if I ran my shower long enough it would overheat every time. I always check the pressure of the system and it is always between 1 and 2 bar. The boiler has a temperature gauge on the front but this doesn't seem to work. The water temperature is not set very high so I do not think that is the problem. The boiler has never overheated due to the central heating being on, only hot water when I'm in the shower.

Any ideas?

I forgot to mention that I am using a mixer shower run from the bath taps.

I have been told by someone that this may be due to my shower head restricting the flow leading to the water in the boiler getting too hot. Is this a plausible theory? Does a low flow rate of hot water through a combi boiler lead to overheating?

I am sceptical of this theory due to the fact that the amount of time it takes to overheat varies, and sometimes it doesn't overheat at all.

In a mixer tap does the amount of cold water flowing affect the flowrate of the hot water? I.e. if I ran just hot water through the mixer, would the flowrate through the boiler be higher than if I ran hot and cold taps through the mixer?

If this is the cause of the problem does anyone know where i can buy a shower head with a higher flow rate?