Sorry to say that I don't quite understand your description of the problem; are you saying that if you're driving the car (>2,300rpm) and depress the clutch the revs reduce so quickly that they fall away and the car stalls. You see a glimmer of the engine warning light when this is happening?
One other possibility to add to the good points above;
With an engine running at >2,300 rpm there's quite a lot of suction on the induction side of the engine, drop the accellerator and the butterly valve closes which turns suction into vacuum. If that vacuum is compromised in any way (ie air leaks into the induction system) the engine may go sufficiently lean as to stall.
How could it be compromised? One possibility is the inlet manifold gasket and another the brake servo vacuum system/hose - if either is compromised they could leak, in-extremis. On other cars I had it was the butterfly valve/throttle(injector)-body spindle end bushes wearing - air is litteraly sucked in through the bushes and makes the engine go momentarily lean.
This may be hard to see and test on the Punto as by recollection the throttle position sensor sits on the end of the butterly valve spindle. You my need specialist help with that.
Certainly if the lambda sensor sees lean (and cannot compensate with extra fuel) it'll show up as an error on the engine management light, as the engine fades away.
Hope that helps.
Jules
PS - I had this happen on a Renault 5 Gordini (non-turbo) I bought in the 80's. I had a similar problem (even at tick-over/died on the overrun). Turned out the inlet manifold had gone porous - a specialist tested it on a bench and painted it with silver Hammerite!
PPS - if you've got carb-cleaner stand the car of the drive and when already warm run it at 2,3-2,500rpm then drop the revs; does it stall? Try again this time as you drop the throttle spray carb cleaner around the throttle spindle, inlet manifold, brake vacuum union (you may need a couple of goes at this, testing a different area each time). If the engine holds on then you'll know you've an air leak. Otherwise stick with changing everything bit by bit.