Out of Memory messages

I keep getting Out of Memory messages, at least 4 times a day, usually more. When I look at the memory status there is usually 1 gig or more spare memory. This is happening on 2 machines, one an 8 core with 3 gigs of memory, the other a quad with 4 gigs.

If you ignore the warnings eventually C1 falls over, but not for awhile. It's more of a nuisance than anything else more serious. Any ideas?

Cheers,

Terry