Strange Image Flubbering/Convergence Problem, Flutters From Dark Picture To Light Picture.

I recently picked up this 50" Toshiba RPTV free from a friend who upgraded to a big plasma, and didn't want to try and mess with this one.

One symptom I've noticed is that the screen will go from being normal brightness to suddenly much darker. I have attempted to re-converge the image with the convergence board I unscrewed off the front of the TV. Although when I attempt to align the colors, it will sometimes be fine, normal colors, normal brightness, and I can get the picture relatively converged, but then one of the colors seems to flubber around. It then seems to switch from normal brightness to darker brightness and the convergence is completely different from what I was just converging.

I haven't done anything other then tool around with the convergence board, and I gave it a well needed dusting.

Thanks for any help,
~Trevor