Today I went to reset my MI424WR today (because it hadn't been reset in a while and the firewall page had hundreds of UPnP entries) and accidently touched the coax adapter. Normally I wouldn't have even noticed, because as I recall, it never was even warm at all, but this time it was burning hot! Do I need a new router? (Also, this is a revision C.)
I also have revision C, and today the entire router got so hot that I could barely touch it. Called Verizon, they said they're sending me a replacement (revision D) immediately. Apparently they have seen these types of overheating on a monthly basis. So, I would suggest you call up Verizon and request a replacement.
Probably good advice ... a general rule of thumb for any properly ventilated consumer grade electronic device is that it should NEVER be too hot to touch (well, unless it's purpose IS to get hot -- like a stove, iron, etc.). It may get warm or even feel hot, but not so hot as to be painful to touch. Even improperly ventilated devices shouldn't get that hot -- they all should have a thermal overload protection inside that will kill power to the electronics before a risk of fire develops (and to protect the electronics from frying themselves).