Don't jump the gun !
Before you go rushing into buying expensive bits of kit change the filters in the system, a blocked inlet filter can cause low airflow and poor adjustment of the regulator will mask this.
Fit new filters to both the inlet and regulator, these will only cost a few quid and will eliminate them as the sorce of the problem, once that is done you can investigate any other issues, my best guess is that the bearings in the AI are giving up but as you would want to protect a new AI with new filters it makes sence to change the filters first.