Hmm it works fine on my 2600 Jr which isn't modded but my six switch which has the etim 2600RGB mod it doesn't even load the menu. The six switch works fine with normal cartridges any thoughts on where to start debugging?
Sorry, I should've paid more attention and read more carefully --- I though you were talking about a simple composite or S-Video mod, but I guess you are talking about this. That is a pretty invasive piece of electronics. My best guess is that the power supply cannot provide enough power to the 2600, the mod and UnoCard together. Apart from trying a more capable power supply, there's not much that I can suggest. Another possibility is additional noise on the bus generated by the mod or the switching power module.
Do you have a Harmony cart? Does it work with the modded console?