big problem with wp 2.8.4 and 2.8.5

i have a big problem. i have a classified site on wordpress and the visitors post every day like 300-400 posts. the problem is that the script has a big consum of memory and blocks the server. i had this problem on 2.8.4 and i reinstall it and now i’m on 2.8.5 and the same problem. my host is one of the top host in my country and i belive them.
suggetions?(also i have wpsupercache with normal cache mode installed and nothing.)
thanks.