2007 Nissan Murano Keyless Entry Going Out?

I Recently Bought a 2007 Nissan Murano which only came with the one key, even thought the dealer has promised to get me a key-less entry remote its been over 3 weeks and i couldn't wait so i purchased two off of ebay, i easily reprogrammed them and they have worked perfectly for the last week then yesterday i had problems getting either to work it would work one hour but not the next couple of times so i guessed it must be the battery's going out and replaced them, neither worked and i then tried to reprogram them one would work successfully and then 30 minutes later not, i emailed the seller of the remotes about the problems and the temper-mental remotes and he suggested it was the internal programmer in the car that was "going out" has anyone every heard of this and if so how much is it to replace?