Ouch !... 1000v mains would be something to see 

I realise digi-Meters are a pain in high noise/RF areas and 'play up' but i can't imagine anything noisy enough in a pin to upset a meter? unless you have installed a custom switchmode that's generating hash everywhere???
or the mpu crystal somehow is generating something that the meter doesn't like, but i wouldn't expect that.

As a matter of interest, can u put a NEW battery in your Digi-meter, and take voltage readings again?
It's be nice to confirm what the hell's going on there..... hopefully 1000v now reads 230vac 

Running outta theories....

Marty.