water to the shower. It seems to heat the water well initially and
provides an adequate flow of water to have a comfortable shower but
then after a minute or two of running the water will go cold for 20
seconds or so before getting hot again.

This can happen if your combi isn't modulating properly, (or is a
non-modulating type). What happens is that the temperature rises and rises
until the main boiler thermostat kicks in and turns off the burner until it
reduces again. It takes a good 20 seconds to relight the boiler when the
temperature recovers (which will be almost immediately).

If the boiler is non-modulating, then your only hope is to increase the
water flow. This might be achieved by installing a wider bore hose on the
shower or a power shower head with larger holes. You could experiment by
leaving a basin hot tap on slightly to see if it cures the problem.

Christian.