intermittent flat spot

Perhaps another tough one.
 

95 CC Lancer 1500  carburettor model.  I have an intermittent flat spot, mostly occurs after backing off and re accelerating or changing up the gears.  The car can run beautifully for days then the problem re-occurs.  I have replaced the carburettor and emission control valve (both second hand) but problem is the same.
Any ideas on how to correct?