Running multiple graphics cards is always a gamble unless they are the same and the drivers are happy with it due to driver and resource conflicts.

I would suggest replacing the existing card with a new one and getting a CRT pc monitor off gumtree or freecycle.

Going from 576i cameras, to a progressive scan encoding and then back to interlaced for your TV is a bit mad, given that progressive scan computer monitors (which will be higher res and so can do e.g. 4-up monitoring) are so widely available.

HTH
Chris