73 914 2.0 have a bad flat spot between 3000-3600 rpm replaced all regular stuff(position switch,MPS,checked timing).please help

if you have replaced the throttle positioning switch, try one of two things.
First, clean the throttle body inside and out.
Two, test the accelerator cable for binding (could be in the tube, could be the peddle assembly.

this occurs frequently and can be misdiagnosed as a flat spot.

Rich

forgot one thing

The dizzy has old (30 year old) grease in between the advance plates (under the points)
Take that apart and clean a re-grease that,,,
when they get hot they have a tendancy to stick so you get no advance, or it might be tight in an advanced position.

Rich

Not exactly hi tech, but had same problem on my 2.0. Removed and cleaned injectors (compressed air 'back wash') and it went away.