My guess would be that the bike has been ridden primarily in the small and middle rings and though they don't appear to be worn, they are. This happened to a friend of mine last year during a race. He had done just as you have and replaced much of the drive train and kept the rings as they looked good. He even did some hard training rides for a few weeks prior to the 48 mile race. At about the mid point in the race, the rings wear hit that magic point where they looked good but would skip under pressure. He had to pedal lightly on all climbs and it was a frustrating day. He thought for sure it was skipping on the rear. It actually took both of us looking at it to see the skipping. Two new rings and the bike is now fine. Just my .02 YMMV