So many things affect ping! Your network connection, your ISP, the server, the time of day, even your PC itself.
If your PC is behaving normally but pings are up, I would look to see what else is running. You may be downloading something (e.g. Windoze update) or someone else on your network may be doing same. If you don't see anything obvious, do a Ctrl-alt-del and look at task manager. Google any tasks you don't recognize to get their descriptions -- you may have spyware or a virus. Just in case, run a full scan with virus and spyware checker(s).
If your whole PC is slow and stuttery, you may be running a background virus scan or similar. Check task manager and sort by CPU, see what keeps popping up at the top.
Also try different times of day. Peak hours for the internet are around 8 p.m. The whole internet tends to be slower then.
If your pings stay up over a couple of nights, it could be your ISP. Call to see if they are having router or DNS problems or something. If you are on cable, maybe they put more people on your local loop (everyone in your neighborhood shares it). If you are on DSL, it could be bad connection to your DSLAM or even wet earth between you and the CO causing ground faults (did it rain hard recently?). With DSL they can tweak things to improve your ping usually at the cost of bandwidth (basically, reducing bandwidth so you don't have so many errors and retries).
Another thought... if you are using wireless, someone may have bumped your antenna. Check your PC and your WAP. Or, there may be something causing interference like a cordless phone or baby monitor, or a neighbor started using the same channel. Try a different channel.