Potterton Puma e80?

I moved into a property in Warwickshire that has a 12-15 years old Potterton Puma e80. We have two showers, one on ground floor and one on the 1st floor. The boiler is on the 2nd floor.
We have been having problems with hot water since we moved in but CH works well. There is low flow in all the showers and taps. There is hot water in the ground floor shower but in the 1st floor shower the behaviour is not consistent. Sometimes we get hot water, sometimes hot water for 1 minute then cold water for 2-3 minutes and again hot water/cold water. Last couple of days we got luke warm water in the 1st floor shower. No problem with hot water through the tap on the 1st floor. Only the shower.
We changed the shower head, but no change. We changed the control board of the boiler, but no change. Called some experts and some even checked the boiler/system. one said it's the temp sensor, another one said all sensors work fine it's electrical problem. One suggested over the phone to descale the system. Any ideas?