OK - I have learned a few things... The new Xorg in 5.5 treats certain things in xorg.conf differently than before. In the case of 4:3 HDTV installs where EDID is not available (I don't think any 4:3 older HDTVs will provide EDID) it is imperative that the DisplaySize parameter is accurate. I also found it necessary to use the options below to make this work. Now the system will switch between ModeLines correctly. See my Xorg.conf below (or the important part of it. Hope this helps some folks.
Code:
Section "Monitor"
Identifier "Monitor0"
VendorName "Unknown"
ModelName "CRT-0"
HorizSync 30.0 - 83.0
VertRefresh 60.0
Option "DPMS" "true"
Option "UseEdidDpi" "FALSE"
Option "DPI" "750 x 562"
Option "NoLogo" "true"
Option "NoBandWidthTest" "true"
Option "NoPredefinedModes"
VendorName "all"
ModelName "all0"
UseModes "Modes_jr"
EndSection
Section "Modes"
Identifier "Modes_jr"
Modeline "1920x1080jr" 86.906 1920 2024 2232 2576 1080 1081 1090 1125 interlace +hsync +vsync
Modeline "640x480jr" 26.718 640 704 800 848 480 490 492 525 -hsync -vsync
# ModeLine "nvidia-auto-select" 26.718 640 704 800 848 480 490 492 525 -hsync -vsync
# ModeLine "nvidia-auto-select" 86.906 1920 2024 2232 2576 1080 1081 1090 1125 interlace +hsync +vsync
EndSection
Section "Device"
### Available Driver options are:-
# sw_cursor is needed for some ati and radeon cards
#Option "sw_cursor"
#Option "hw_cursor"
#Option "NoAccel"
#Option "ShowCache"
#Option "ShadowFB"
#Option "UseFBDev"
#Option "Rotate"
Option "ModeValidation" "NoVesaModes, NoEdidModes, NoXServerModes, NoPredefinedModes, NoDFPNativeResolutionCheck"
Option "NvAGP" "0"
Identifier "Card0"
# The following line is auto-generated by KNOPPIX mkxf86config
Driver "nvidia"
Option "XvmcUsesTextures" "false"
Option "UseEvents" "true"
Option "UseEDID" "FALSE"
VendorName "All"
BoardName "All"
# BusID "PCI:1:0:0"
EndSection
Section "Screen"
Identifier "Screen0"
Device "Card0"
Monitor "Monitor0"
# Option "metamodes" "640x480jr +0+0; nvidia-auto-select +0+0"
DefaultColorDepth 24
SubSection "Display"
Depth 24
Modes "640x480jr" "1920x1080jr"
EndSubSection
EndSection