I still have this problem as well. It is a bit better in the latest hot fix (3), but basically my ram usage slowly (and I mean really slowly) climbs until its at ~95% usage; if it manages not to crash I just get really terrible stutters that basically make it unplayable..
Seems to happen in both single and multiplayer for me but I'll need to test it a bit more to see.
8gb Kingston HyperX ram
EVGA 660ti (2gb vram)
i5-2500k @ 4.0ghz