Cant reset resolution on 2nd monitor (HDMI)

I changed and accepted to the recommended higher resolution for my 2nd monitor (Samsung Plasma TV).  It worked for awhile but the TV does not seem to work reliably there and I have not been able to make it work again.  Further, now when I plug in the HDMI cable to the TV, the primary monitor display goes blank (it can't support the higher resolution) and the display on the TV does not work either, so there is no way to reduce the resolution for the HDMI output to the TV.  I understand that the higher resolution for the TV is now in the registry so as soon as I plug in the HDMI cable, the system switches to the higher resolution.

Is the only solution to edit the registry?  If so, can anyone point me to the correct entry?