Wheel Balancing Screwed Up?

Earlier this week, my mechanic, who's been solid as a rock for 20 years, removed my left front tire to investigate air leakage. He rebalanced it before reinstalling. I left, and as soon as I got up to 55mph, the shaking in the front end was pretty serious. I could not go back at that point due to scheduling problems, so I dropped down to 45mph, at which point the shaking stopped. It'll be investigated this Tuesday. Yesterday, I had to drive the truck again, and there were no symptoms. Around here, people will occasionally get a chunk of ice stuck behind the wheel, and it'll cause this kind of vibration until the ice melts. For various reasons, this doesn't seem likely in my case. What else might cause an intermittent thing like this? I mean...if the wheel's not balanced right, the weights aren't moved by elves at night to make it correct.