I have a Cervelo P3 with Shimano Di2 shifters and 2 sets of bar end shifters, one set on the aerobars and one set on the handlebars. They are set up so the left buttons put you in a higher (easier) gear and the right buttons put you in a lower (harder) gear. I think the front chainrings are 39/53 and the rear cassette is 11/30. The battery is fully charged, and the problem just developed in the past few weeks.

The problem is that for the first 10-15 minutes of riding the front derailleur will not shift from the small chainring to the big chainring. The rear derailleur shifts normally, and when the front would normally go to the big ring the rear goes from the 7th cassette to the 5th (I think). In any event, it goes where it would if the front went to the big ring. Somehow it knows the front did not change, because additional shifting results in a 5-6-7-5-6-7 pattern. After a period of riding, maybe a certain number of shift attempts, the front goes to the big chainring and everything is fine from that point on. But the next ride it's the same problem.

If I start in the big chainring, all is good until I shift to the small chainring, at which point it again takes the extended riding period described above before the front will shift back to the big chainring and normal operation returns.

Any ideas? Thanks in advance.