The FEC-100 definitely does "yo-yo", but it is a controlled "yo-yo" just like when an oven turns its heating element on and off.
I imagine you would see a "yo-yo" effect on any cooker that used something like a fan/pid as the fan and fuel cycle up to get back to the set-temp.
I personally do not believe this is a bad thing when it comes to producing a great product.
Pyle, have you looked at the FEC-100 for your restaurant? NSF etc., and I have cooked some ridiculous amounts of food in it as well as run it for hours on end. I love that cooker. Best BBQ investment I ever made except for maybe my subscription to this forum. (yeah shameless plug, but we all know this place rocks)