Ph Probe problem

I recently replaced my lighting system and now have my fluro's run from two
electronic ballasts which I have placed remotely in the aquarium cabinet.
The tubes are suspended from the inside of the hood which rests on the
glass. the wiring is therefore also contained under the hood. 

As I am not running a sump my Ph probe is also placed under the hood. I have
it positioned as far away from the wiring as practicle.

The problem I now have is that the lights seem to interfere with the Ph
reading. Whilst the lid on the hood is closed I am getting a reading of
close to 6.9, however as soon as I raise the lid the reading plumits to 6.5.
If I turn the lights off the reading also drops to 6.5. Using a ph test kit
my ph seems to be around 6.6 (the kit is not very easy to read). I have
checked the wiring and nothing is coming in contact with the wiring to the
probe. Obviously there is some electrical interfernce here that I must deal

Would insulating the lighting section off to the Ph probe area work and what
sort of insulating material should I use. Has anyone else had simular
problems? Any advice would be greatly appreciated.

In sunny Adelaide, South Australia.