Have you checked your chain tension?
I'd say chain tension too.
If you have had good luck in the past with the 20-50 then I guess thats not the problem. I tried it two summers ago and it was fine when the temp was in the upper 80's, but anything below that the bike did not want to shift. I went back to 10-40 year round.
I did some rides with my 20-50 and it the bike would shift terribly in the morning at 50 degrees and great in the afternoon at 80 degrees.
To me, 60 degrees isn't warm enough to start using 20-50...but thats just me.