Have they identified a problem cylinder(s), or is it just random? Have they checked for fuel contamination? Does it have a collapsed engine mount? If all that stuffs' been found to be OK, I'd check cylinder compression/leakdown. It doesn't take much time, and can tell you a lot. If no problems are found, I'd pull the fuel rail and check for a pi**ing injector. I feel stupid recommending these steps, since I'm sure they've already been checked, but you never know.