Host instance memory usage keep increasing

We are using Biztalk 2006R2 in Windows server 2003 64bit. The purpose is transferring files between systems.

We found that the Biztalk host instances (sendhost, receivehost, trackhost, applicationhost) memory usage is keep increasing, without dropping down, even the file transfer has been completed.

Just want to know whether Biztalk use garbage collector to release non-used memory or not.

If yes, where I can set the frequency to make the garbage collector run more frequent?

If no, why the memory used is not released?

We found that if the memory increased to certain level, some messages are pending in Biztalk without sending out. We suspect it is due to the memory reached the throtting value such that it does not send out, until the memory drop below the throtting value.

Thanks.