I have started facing similar problem recently. (Although I didnt add anything at all to the hardware)
What happens is that there is no problem during the boot time.
However, as soon as the Windows loads (after the login screen) there is constant flickering sort of thing. The monitor blanks outs, and comes back.
- very similar to the thing you observe, when you change the resolution and refresh rate of the monitor.
And this thing keeps happening, with increasing frequency, day by day.
The weird thing is - when I turn on my TV card (internal one). The flickering doesnt happen.
really WEIRD.