Total Cache Buffer diminishing

I was wondering why the “Total Cache Buffers” on my 4.11 Server runs down to almost nothing and then I start to get a message that the system can not allocate memory. I have to down the server and restart. Once this is done, the cache buffers seemto be fine for a while and then it starts again after about a half a day. I have 196MB memory. Orignal Cache buffers on is at about 48,480 and after a reboot the Total Cache Buffers is at about 40,500. Over the last few days, it seems to have gotten worse. I just Rconsoled in from home and with nobody even working on the network, the Total Cache Buffers is still falling off. What happening?? I have other server on the Wan and they seem to be fine