2001 Hyundai Accent Vibrations

I work quiet a long time on my car and know it from a to z but this one drives me crazy:
Around 4 months ago my car starts shaking on the front passanger side at speeds over 55 mph, only on acceleration. And its not all the time, sometimes it doesnt show up for a whole trip to Austin (160 Miles), sometimes it comes after 20 Miles, sometimes it starts immidetly. I assumed the tires first, balanced them twice. No effect. Alignment - no effect. We changed the brake pads and checked the discs, berings and the cv joints - all in best working condition. I changed the struts before that starts so I guessed it comes from them. Couldn't be, they will vibrate all the time like the cv joints. Do you have any idea where else that could come from. I am out of answers. Thanks