Hi Mike,
Don't feel alone, I too have this problem at times. The last was playing at our church for a Memorial service. When I first started I was completely relaxed, but then the old foot started to shake. I just take my foot off the expression pedal for a few seconds, pay more attention to the music while relaxing my foot on the floor. It seem's to pass quickly. I believe that it is do to the fact that we don't play in front of people on a regular basis and even through we practice and try to relax the stress seem's to go to the foot on the expression pedal. I use a Technic pedal, which to me is quite high off the floor and the foot is not supported very well when using the pedal, which I feel contributes to the problem. Hope this helps - happy playing - if the foot trembles - just give it a few taps on the floor.
Best Regards,
Dave