Main Fuel Screw and Idle Speed

[ally406]

I have an odd problem on my 406 with Bosch and TD04. Car is running great and I'm now trying to dial in some more fuel. I'm using the idle adjustment at the back of the pump to bring the idle speed back down after turning the fuelling up (I usually go with 1,000rpm since that's decent with cold veggie). However, often when I start it, or dip the clutch at a junction, it will start to chug at 500rpm. A blip of the throttle and it will usually come back to normal idle speed. This will persist until I return the fuelling and idle adjustments back to standard, so I'm in the position of not being able to add more fuel   Only other adjustment is a ground LDA, otherwise it's an unmolested pump. Anyone seen this before?