To me it sounds like you might have a worn or even burned clutch disc. The shuddering you describe is the clue. Excessive slipping of the clutch can burn the disc and cause what I know as "chatter" no matter how many miles you have on it. It can also cause overheating of the flywheel and pressure plate. The flywheel can develope cracks and overheating of the pressure plate can ruin the springs requiring replacement of both.

I went 120K miles on my 300ZX clutch before replacing it and I waited a little too long. It had started slipping and by that time and I had to do the whole schmear... new flywheel, clutch plate, and pressure plate. The throwout bearing was a given. I'd suggest getting it looked at by a competent mechanic and probably go ahead and at least replace the plate and resurface the flywheel.

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'05 X3 3.0i Alpine White/Gray Leather/Alum. Trim
Sport/Steptronic/Servotronic/Panoramic
Prem. Sound/Staggered 18" Style 114 Y-Spokes