3G iOS4--Yahoo Push Mail not Working

I upgraded to iOS4 a week or so ago--now, for the last 3 or 4 days, my AT&T Yahoo Push Mail doesn't push. I can receive mail if I manually retrieve it, and all other phone functions seem to work properly. I have tried several times to shut down and restart the phone with no success thus far (using wake/sleep AND home button). This has occurred two or three times in the past 16 months of iPhone use, and seems to mysteriously correct itself after a week or two. In looking through the Apple Support Forum, there seem to be lots of people with email related problems but no apparent solutions. Does anyone on this forum have an issue similar to mine, and have you discovered a cause or solution? Thanks for any information--