300mm f/4 L issues with 5DII body

Hello,

My much loved 300mm F/4L IS has started developing some problems when used with on my 5D Mark II.  At first, all shots were being over-exposed by about 2 stops. Shortly afterwards (a few hours), I started getting an error message on the body saying that it could not communicate properly with the lens. When I use the lens now on the 5DII, its behaviour is pretty random, sometime it works fine, sometime it overexposes, and sometime it just throws the error.  The undesirable behaviour seems to happen more when the ambient temperature is low (5-10 degrees C).

What baffles me is that every other lens I have works fine on the 5DII, and the 300mm works perfectly on my wife's Rebel T2i.  It's only the combination of the 300mm and the 5DII that causes an error.

I'm about to contact Canon for service, but I'm not sure whether the problem is the camera, lens or both.  I'd rather not have to send both in if I can avoid it.

Has anyone experiences something similar?

Thanks!