I can give you a 10 page rambling about my problems with the Linksys WRT54GS. These are the things you should do that worked for me. Sadly all but one of these things were temporary fixes for me, the most recent one still works but I still wonder how long it will work for. FYI, if you connect your PC wired and wirelessly to your router and you still get ping from the router it is probably not interference.
1) The simplest thing to do is just pull the power cable to the router for one minute and it might solve the problem. Now what I did the last time I had high ping was disconnect the routers power, then the modem. I waited a minute or two then reconnected the modem. Once the modem got a connection to the internet I connected the router and it solved the problem. According to the Linksys tech I might not be getting constant voltage effecting the operation of the modem and/or router.
2) Make sure your firmware is the most up to date, even if it is download it again as your current firmware might be corrupt.
3) Lowering the MTU settings worked for me a few times. In my case you need to take it out of automatic and into manual to do this.
4) Consider changing the channel your router broadcast on.
5) Plug your PC directly into your modem and do a ping check. Either with the Command Prompt (Ping yahoo.com for example) or use a speed tester like broadband reports. Your ISP might be giving you the lagg.
6) The final thing you might consider doing is re-setting the router to its factory settings. Look for the small reset button on your router and hold it in (Probably for 30 seconds) and the router should be re-set. Consult your manual since the way you do this might be different than it is for my router.
I hope this works for you. If none of this works and the problems come back consider trading in your modem for a new model. Linksys offered to give me a new WRT54GS if they can't solve my problems. If I have high pings next time that can't be solved by the mentioned steps I will consider doing this.