4.8 clutch shudder

[norv struckman]

I have a 2000 short bed single cab chevy pickup with 160,000 miles on it. The clutch pedal started sinking thus loosing clutch disengagement which required double clutching to save the trans synchros. Shortly thereafter the clutch wouldn't slip for a smooth application of power in forward or reverse. Had a local shop replace the throwout (showed wear an loss of thickness) pressure plate and disc and had the flywheel ground, everything through NAPA. Clutch free play is back but it still shudders, so NAPA sprang for new parts on their dime but added a new flywheel. Now the clutch shudders worse than ever! Thinking of taking it to the dealership for original GM parts, as all I can think of is substandard aftermarket parts from NAPA. Advise, thanks, norv.