at 2500 does it stay rough from then on as high as you rev it? is that part or WOT or cruise. I would measure the vac & fuel psi & timing at that exact point when it starts to act up. Yes do a compression test & maybe a leakdown & try the new plug first & see if the new plug straightens it out for awhile. Describe the roughness (shakeing/missing). EDIT if it dont continue to be rough from 2500 on up, how much more RPM does it continue to run rough?