I'm on my second Epic 4G, got a used one off eBay about a month ago when the screen on my first one died.
On my first one, I rooted it, upgraded to Clean GB, then eventually moved it to cm-10.1.3.1.
The used one came stock, but ran extremely slowly. No problem - I've seen that happen before with my old phone (I call it sloth mode) and knew that installing a new ROM would resolve it. Just to be conservative, I bumped it up along the same path I did before, Clean GB, then cm-10-1.3.1.
Here's the thing. After installing a new ROM, both phones would operate normally. But no matter which ROM ran on the old phone and the replacement, *eventually* it would slip back into sloth mode. I don't mean sluggish, I mean crawling, even during the bootup animation. Even the CWM recovery menu was sluggish.
When I ran Clean GB on the replacement phone, I didn't add more than 2 or 3 apps to it after installing. But it still fell back to sloth mode after about a week of use. After installing cm-10.1.3.1, it only lasted a 3-4 days before sloth mode returned.
Before changing ROMs, I routinely did a factory reset, cleared cache and Dalvik cache.
Of course a virus is a possibilty, but having security apps installed and running didn't seem to make any difference. Could this be a rootkit-class issue?