Strange things have been a'happin' with the rev counter on my 33.
When flooring it in second up to about 4500 the needle "sticks" at about 3000 then jumps up to where it should be. A similar thing occurs when decelerating, except it will stick, and jump up, then move down smoothly.
This also happens in third.
I have a problem with my rev counter too - when the revs are increasing slowly past 3000rpm, the needle starts to waver around, wobbling up and down, then it settles down again and points at the correct reading.
I have an electronic kitset rev counter (see http://www.axys.co.nz/ben/alfa33/TACHO_IN.JPG ) which takes its reading from the low voltage side of the ignition coil. It behaves normally at the same time as the factory rev counter is freaking out.
(I suspect the kitset is faulty though, because it reads 0 when the engine isn't running. Everbody knows all old Italian cars do 500~600rpm when the engines are turned off - just check their tachos...)
The rev counter in my spare parts car works normally, so I must try swapping the gauge clusters from one car to the other to see if that helps find the fault.