I work as a freelancer, so this question pretty much defines my life. In my market, due to holidays and budget years ending, I pretty much have a vacation from mid-November through Valentine's Day. During the rest of the year, it's frantic feast or famine.
While a workplace situation may be somewhat different, I tend to be "productive" during my downtime by studying industry trends, working on personal pet projects to stay on my game, and do a lot of the "personal development" type activitites that keep me ready to pounce when the feast approaches again.
The reality is that in my world (due to market conditions) and yours (poor management), consistency in the workload may be a pipedream. I've learned to live WITHIN this paradigm. I've always got a couple things waiting on a back burner for when there's a slowdown. Pro-bono work for non profits, taking more time for professional networking, online classes or even reading a book that helps me run my business more effectively.
While it may seem like downtime, I fill it with--and here's the secret--applicable skill-building tasks and activities--instead of surfing the 'net and beating myself up for being "unproductive".
It sounds like self-management isn't necessarily going to solve your workflow problem, but perhaps viewing a slowdown in the workload as an oportunity for self-development with workplace related skills may be a better solution.