If it were me I'd look at three potential culprits:

1) Trapped MAP hose
2) Ignition issue ; I'd say it's either a rotor arm issue, especially if it is a pattern part and not an OE brand or it is a spark plug issue. They may be new but I'd throw some NGK BCR8ES plugs into, and only from half decent motor factor (and ensure they are made in Japan).
3)TPS issue. Check it is working as intented. IIRC it has a switch in for full throttle operation and idle.

Regarding the plugs I would check them. On an old Triumph Stag I changed just about everything on the ignition. On normal driving it was fine. When giving it the beans it would splutter, misfire and backfire out of the exhaust at 3.5k and above. After a while the problem came and went. People said the engine was overfuelling. The day I changed the spark plugs a year later the problem went away completely.

The first set of plugs I got from Halfords and another motor factor. They were NGK BP5ES' but they could be fitted according to NGK. I checked the gaps a few times as well. The second time around I used BP5Es from a known Triumph Supplier. It never idled so good and it passed the emissions first time on all counts.