baxi instant 80he hot water fault

[srplumb]

annoying hot water fault. have a baxi that works fine at all hw outlets except the shower. its a bar mixer and is intermittantly working, sometimes working perfectly other times not working just cold water flowing. but whilst the shower is running when not working, if i open the basin hot tap it brings the hot water on at the shower, then turn the basin tap off and the shower will run hot long enough for a shower.

seems to be a flow/pressure fault but cannot pinpoint anything. any ideas would be appreciated cheers