Sticky tach

Recently put new glass and bezel on my old tach (which was working fine, just had broken glass) and now the needle's motion is very sluggish--it tachs up slowly, then either hangs up or returns to idle speed very slowly. I can't see any physical obstruction to the needle on the dial's face, so I assume the problem is internal, or at least below the tach face.

The cable seems to be ok, I greased it and it doesn't have any bad kinks.

Did I mess something up when I put the new bezel & glass on? Any suggestions (short of a $250 rebuild)?

Thanks!