I have had rare instances in which regardless of booting and hitting reset, the Jag screen comes up and then turns red and stays even after hitting the reset a bunch. But once taking the cart out and reinserting it then it's fine.
Ad for your rainbow screen lasting forever, I have not seen that, but have seen the black screen with yellow flickering last for a long time!
I am having a problem with some of the roms working. Raiden is an example, no matter how many times I load it, after it is done, the screen just goes to blue. Any of you have that issue? Even after retrying and resetting and re inserting the cart no matter how many times I flash it I get the same result. I have had some that at the end of the flashing it turns blue then quickly pops over to the working Jaguar screens and the game starts, but Raiden and a few others I can't remember just seem to turn the screen blue and will never load. Could that be a rom specific issue?