My cbr 125 feels very sluggish and starts twitching on mid range, starts exactly on 6k and stops on 7k?

Before 6k and after 7k the acceleration is good and quick, before i had problems where the engine would just cut out when hot so i changed the ignition coil with original honda one but this problem is still there, i just don't know where the problem could be, the carb has been all cleaned and a bigger jet to match the new exhoust was fitted…

Check the timing.

Check the burning condition of the spark plugs. Guessing it may be running too rich but that's a hard guess.

Check the ignition timing and the valve tappet clearances

Changing just the main jet is not enough to properly re tune a carb to match a new hi-flow pipe! The next step is to try different slide needle positions by moving the cir-clip on the slide needle up or down in the set of grooves it has (moving the clip down enrichs and moving the clip up leans the mixture) if you can't find the spot that fixes it doing that then you will have to start trying different Pilot jet sizes and then again trying each slide needle position for each pilot jet size you try until you find that perfect combination that results in that dead spot going away without creating any new dead spots in different RPM ranges!

Note to the other answer submitters Timing and or valve clearance will absolutely never cause a problem in only a 1,000 RPM portion of the RPM range!