I've worked for 4 companies in 5 years (avg of 15 months per employer), and it doesn't seem to bother new employers. I can always spin it as "we finished our one year roadmap and I'm looking for a challenge that my current company can't provide." Besides, at each company I've worked for, the average lifespan of a developer seems to be about one year on average, 2 max.

If and when it does become a problem (i.e. interviewers start saying "this pattern looks disturbing"), the worst that happens is that it gets harder to get new job offers, and I stay at my current job a little longer, knowing that I got an average of 19% per year in pay increases while the average Joe (if he's lucky) got 5% per year, or in the OP's case 2.5% every other year.