I brought this issue up last year around this time and it was, bluntly put, ignored. I have a WRT54G and noticed many oddities - ping times would be at some normal level and increase about 5 times normal then return to normal level. This would repeat for Interface run duration.
With my Motorola SBG6580 cable modem/wireless router it would do same, but, worse leading to it eventually ceasing to work leading to needing a reboot for it as it would no longer route any net traffic for anything to any computer attached to it.
Netgear WGR614v9 exhibits cyclical massive increase in ping time and after about 3 hours would cease working with Interface to remote servers but would be usable for other applications.
Using any of those with a cable connection vs wireless resulted in normal operation. I began digging with wireshark and saw some interesting metrics, but, nothing I could point to and say this is the problem.