MAF implausible 16485 ( not the MAF )

[redeye]

Hi all.
Tried a new MAF in my mates AWX TDI also tried mine values are good on all 3 sensors.
So we have intermittent code. 16485 MAF implausible signal. This can be corrected by running without the MAF then reconnecting the MAF and it's OK for a couple of days. Had a look on Ross tech
Air leaks ( none ). Air filter ( new ). Plugs (look fine).
The next one is ground spot corroded? Has anybody got any tips on how to find the ground spot or anything else worth checking?
Many thanks Nige