Help FZR250R 3LN5 rough running and slow to rev

Hi everyone, sorry to resurrect an old thread but im in need of some help regarding the running of my bike following completion of new engine install.

When i took the bike in for its warrant of fitness, the man doing the test found that when he brakes heavily the bike stalls, and is subsequently quite hard to start and keep running. It will crank and will start if you hold the throttle wide open, but if you let go of the throttle it stalls immediately. to get it back to running correctly you must hold the throttle wide open, turn on the choke and hold it there for maybe 15 seconds then it 'clears up' and will rev cleanly again, letting you turn off the choke.

If i start the bike and let it idle in the driveway it will idle nicely for maybe 15 seconds? then it sounds like it drops one cylinder, following this the idle drops very low and it will stall a few seconds later.
However when im riding the bike it doesnt seem to have any issues? Maybe if you crack the throttle open very aggressively it will have a small .2 second stumble then pick straight back up. Bike revs out smoothly all the way.

Issue only arises if you brake heavily or if you try and leave it to idle on its own. If you blip the throttle every few seconds it doesn't happen.

Sorry thats lots of information, but any help would be appreciated before i just go and pull the bike down again.

In some good news this is the only problem to resolve and then itll be right back on the road ready for another awesome summer.