Shower Power Boosters will always solve problems of low pressure and fluctuating pressures.
But your electric shower or combination boiler may simply be faulty, you may have a blockage in a shower mixer, it could be an air lock, your service pipe to your home may be inadequate.
A WRightChoice Domestic Water Pressure Testing Kit for £14.99 will allow you to check pressures when other taps are opened, check for corroded/blocked service valve. This test kit which allows both static and dynamic water pressure tests to help identify the problem.
Some plumbers do a static test and ignore a dynamic test but most simply carry out no testing as they regard whatever they do is at the customers risk and if it does not work the plumber expects you the customer to pay twice.
Static pressure testing is a very simple quick and inexpensive process and you do not need a plumber. Just unscrew your shower head and screw in a WRightChoice Static Pressure Guage.
For dynamic testing you will need to cut the feed pipe, insert the 15mm tee, attach the shower hosed supplied in the kit, attach the guage. When you have finished testing, simply, pop in the blank, and leave the tee in place.