Crosspicking is easy if I stick to a roll that doesn't require me to skip a string. Like an up/down roll (forward/reverse roll?)
It's the string skipping that trips me up.
Yes, I have the same experience with Beumont Rag: It all sounds pretty good except for that crosspicking.
Every once in a while, my crosspicking sounds pretty good, and it's because I'm relaxed, thinking about something else (like mreisz says) and taking full, comfortable pick strokes. But when I start stressing over it, it all comes crashing down.