Novice Shower/Heating Question

[Doddles]

I've tried a search but nothing that exactly matches the issues we are experiencing so I'm hoping the collective can cast some light on an issue we have. I have a broad idea of what it may be but I admittedly have very little idea of plumbing/heating systems.

Our house is a typical modern (1999) detached with a gravity fed thermostatic shower in the en suite, to which a pump has been retrofitted in the loft at some point to make it more powerful (which it is and it's excellent). However we have an issue whereby the pressure and temperature of the shower drop off very quickly (2 mins) and the pump makes some awful straining noises. This can be rectified by turning the shower off for 30 seconds which then buys another minute or so of normal performance.

This issue only seems to happen when the central heating is or has been on and I can vouch for this as we had the same issue last year but not over the summer.

Can anyone can offer any advice on a) what the issue is and b) solutions to rectify it (that aren't turning the heating off...) so we can try and solve the issue with a bit of an understanding.

Thanks all.