OK, to begin with. I haven't started to trace the bug yet, I just want to confirm if I am the only one or not.
I use MMC 5 to install all mods and handle different instances of MC.
I have in MMC set Java up to use up to 768 MB.
If I play MC 1.7.2 with any 1.7.2 (any I tried, starting back at some late 900er) forge version and Optifine 1.7.2 HD (no other mod installed at that point) after some time (usually several hours) I recognize that my free ram has significant dropped until it is running out at some point. (which results in a very laggy game play and very low frame rate)