Cavitation Air Bubbles in Hot Water Circuit and Fitting Venting Valve All aspects of plumbing questions and answers, help, tips and information

[Notajock]

I'm renewing a shower pump in main bathroom on 1st floor of 4 floor Georgian townhouse. It has a vented system, CH header and HW and CW feeder tanks in roofspace, and Vaillant boiler and two cylinders in the basement. HW is pumped around the whole house, to the roof and back in 28mm copper by a grundfoss circulating pump fitted in the basement just above the HW cylinders. 
Installing a timer on this pump a few years ago seems to have been associated with a lot of air going around circuit, on intermittent basis. This can be heard in the walls and causes sputtering in the top floor taps.
A couple of Grohe mixer- pumps in the house have either failed or need to self- prime on regular basis.
I'm concerned for the life of the new pump (Stuart Turner Monsoon twin positive head 2.0 bar) under these conditions, especially the reed switch that triggers it.
Can I fit a air venting valve or similar to the system to cure this?
Any advice gratefully received!

[Notajock]

Actually I can't strictly say that cavitation is happening here. There are a lot of air bubbles circulating around, some of the time.
But if the existing Grundfoss circulation pump has coped for 10 years under these conditions, would not a well built pump like a Stuart Turner survive?
Any thoughts or opinions gratefully received.