It does have a keyed lock though.
There's a couple of different possibilities. Either the key is starting to take a dump, the ant is loose or not getting the proper amount of power.
There are special tools, not Benz specific, that will test the strength of the radio signal from the key. I would start with that. I honestly don't know where to get one, as there is a guy in the shop that has one that I use, but I'm sure you could ask around for one.
Eric