Pro Tools 8.0.5 Recording Problem

Hey all. I've been using Pro Tools LE 8.0.5 with the Mbox 2 for quite a few years now. Just the other night I noticed that while tracking guitar to a click I was playing perfectly in time but my riff was laying down off the grid for some reason. Has anyone else experienced this and what did you do to correct it? I tried reinstalling my os and all audio software which corrected the problem for a few hours then it started acting up again. Like I said I have used this exact setup for close to 6 years now with no problems. Thanks in advance.