Ahhhh. That might explain things.
Windows likes to have considerable free space on the system drive... Its needed for swapfile expansion, temp files, etcetcetc, so its entirely possible that you can only use a couple of hundred MB of that 850Mb.
And the history folder can actually get quite large, so you are proably running out of assigned room and its automatically clearing some/all of the history to maintain a minum free space level.
One easy way to fix this would be to assign the cache folder to a different drive with more free space.
<b><i>The Very Hungry Caterpillar</i> - George W. Bush's favorite childhood book.
Note: This book was first published a year after Mr Bush graduated from College.</b>