You've done a lot of work here Graham and unfortunately this also complicates diagnosis when there's a problem. It's always easier to make a change on one thing and then check to see if the "fix" worked. Attacking many different areas at the same time can really task your ability when an issue shows up. So, lets talk one thing at a time.
New Valves- I'll assume that the seats were cut in the cylinder head and the valves shimmed to the maximum clearance. I say that since new valve jobs need to seat in and if the valve clearances are set to the tight side and not checked in the first 100 miles, they will probably be stuck slightly open. So in this case check the valve clearances to rule out any issue due to tight valves.
intake manifolds- Are they properly aligned? you can't just remove and replace the maniifolds. There is some wiggle room on the mounting screws manifold to rubber manifold boot. There is an alignment tool for installing the rubber manifolds. What to do now is to test for an intake leak. I prefer a small propane cylinder (torch size) take the nozzle off the end and put a piece of rubber hose over the end. Start the bike, get it warm, then start leak checking the intake from the bottom side up. Propane is heavier than air. You only need enough propane coming out of the end of the hose to just feel the cool effect on your cheek. An increase or decrease in idle speed tells a tale.
I don't think it's a choke issue. A failure in the choke will stall the bike for sure.
I still think it's a transition port issue. You're problem is right where the idle circuit is being taken over by the main circuit.
Although, I wouldn't rule out a bad plug or lead or coil, but if somethings off with those three items you're usually dead on that cylinder above off idle.
My money is bet on the transition ports.