If you're using 87% of your guaranteed memory at any given time, there IS going to be a problem. I'm not sure why you think there wouldn't be. You're going to have spikes in usage, things are going to slow down, and cPanel is going to start killing off processes.
256mb is not nearly enough RAM for cPanel. A fresh install with no loads, no sites is going to be right around 200mb on its own. 512 is about the absolute minimum I would ever recommend. If there are a considerable aamount of sites on your VPS, 1gb is probably around the minimum.
Of course, if it was working fine before and all of the sudden stopped, I don't know what to say. However, I seriously believe a RAM upgrade is in order. In fact, you should have just gotten one for Christmas.