[urq] Intermittent stumble

[spot.ontheroad]

Hello guys. Any ideas what may be causing the following symptom?

Engine at normal operating temperature
Accellerate hard and it pulls like a train.
Air fuel gauge suggests rich mixture under accelleration. All is well.
Lift off the pedal to change up a gear and accellerate and the air fuel gauge shows lean mixture and she stumbles and doesn't want to go.
Lift off the pedal and take it easy for a few yards and accellerate again and mixture is rich again and she pulls as she should.
If I stay on the gas and hold it in a gear it remains rich as it should do which suggests to me the fuel supply per se is OK (*)
It's making full boost as normal (according to the tell tale on my test boost gauge) but I am not sure if it is doing so after the upshift in the lean patch because of course the tell tale has hit max boost from the accelleration in the previous gear. And I can't drive, watch the A/F and reset the boost gauge between changes.
A couple of weeks ago I believed it was a simple fuel supply problem and changed the filter but it made no difference. 
The pump is not noisy . 
The pre-filter sediment trap is clean but does have a bubble or two bobbing around when the engine is running but I can't see this should make any difference.
I have not found any air leaks on the intake side. The engine stumbles if the dipstick is removed while idling suggesting no air leaks. Mixture at idle is bobbing between lean and stoich as I would expect.

The engine is  2168cc WR 
CR = 7.02
1bar wg spring and zener diode

I'm sure it's fuel related despite my comments at (*)

Any suggestions gratefully received. Many thanks in advance. Look forward to hearing from you.

Keith

'87WR