Thanks for the comments Scorpion !
With regards to your answers, the powerboard for the DVR & 2 cameras is on the same one but has other items connected such as a lamp etc so I dont think it is a power supply problem plus another camera is remote and plugged into a completely seperate power source.
The DVR is in a loft area so I accept the conditions might not be perfect my theory is kept running it would generate its own heat to "keep warm" I accept your comments though but add the problem occurs within 5 - 30 mins of running the system so the temp difference is probably not great.
Would the length of a co-ax cable connection make a difference to video loss perhaps ? I have about a 20m length, 15m length, & 10m lengths, as I say, when running the image quality is very good ...
Thanks again for your input.