7yr old Baxi Combi Boiler Pressure Problem

[Bdc72]

Hi, my Baxi boiler pressure is fluctuating. It can be at 1.5 bar and I turn the heating on and it drops to 1 bar. Or I can switch it on and it rises to 4 bar after being on for a while. The bar also drops when its been off for a while suggesting a leak. I’ve checked all the rads and no leak. I’ve opened up the boiler and there is no leak inside? I’ve been topping up and bleeding the rads to maintain the pressure but keep getting an e11 error message. The boiler runs perfectly when the pressure is correct it is just the change in pressure. Any advise please?