PipL

Hi all.
A few months ago my F VVC went into limp mode, would only rev to 5500rmp, bought a cam sensor as suggested by SAWS tuning when i sent live data to them.
bought a lucas cam sensor for £23, fitted all was good. 2 weeks later back to 5500rpm, checked faults with Pscan and nothing, nothing before either. advice given that the VVC mech was sticking and needed to be stripped down. spoke to a well known MG mechanic who said they had never heard of a sticking VVC mech and suggested to replace the cam sensor again but buy off Rimmers so I did. back to 7100rpm happy days, tonight 2200hrs, back to 5500rpm but earlier in the day was 7100rpm. again Pscan, no faults and no change in the cars driving except lower revs.
Any ideas please, pulling my hair out and didn't have any to start with....