Can any body help me to diagnose my Baxi 105e please?

Hello,
I have problem with my Baxi 105e. I had replaced diaphragm for DHW and it worked fine but now (after few months) it developed something new.
CH works perfect but DHW has its own life. Sometimes water was nice and hot but some times it was lukewarm. Up until now while using hot water (shower) I flushed toilet or release cold water for a while from other tap (pressure of hot water drops). After I closed cold water tap or toilet cistern filled itself up (hot water pressure goes up to normal) then temperature of hot water risen up (also option to switch to DHW only helped for few times). It was doing the trick up until today now its always lukewarm or some times gets bit warmer for few seconds. Pressure is ok, micro switches seem to operate correctly. fires up nicely, radiators hot as it should be. I have no LED’s blinking.
Please help me so I will know if I need new boiler or else. Currently I have to boil water in kettle to get some hot water to bath my 16m old son.
Regards,
Iggy