Shower waste pump cutting out

[debzisme]

My elderly father had a disability shower unit fitted which had to have 2 new waste pumps in the first year through warranty claims. Now 2 years on and onto its 3rd pump that a local plumber installed and again weeks after is only working for so long then the tray is filling up with water and having to be scooped out! Plumber has been back but it always seems to work fine when plumber comes then never works first thing in morning when in use. Also it seems to work for longer at a cooler temperature. Any ideas?

We've tried 2 things and one of them must have fixed it as it's worked perfectly for the last few days. We've moved the control box out of the cupboard so it doesn't overheat and have turned the pump down so it's taking the water more slowly. We think it could have been taking in air and damaging the pumps. So far this one is ok as it's been shutting itself down.