1997 Vauxhall Corsa Cutting out

My P reg automatic Corsa cuts out when slowing down and stopping, it's blocking out diagnostics machines, could the reason it's blocking out the machine be the same reason it's breaking down? Mechanics are baffled over this! We did change the auto gearbox ECU and nothing changed other than it getting progressively worse. My other half threw out the old one and I've been told that the replacement one needs to be programmed, yet the car still works, except for the warning light coming on while I'm driving and the car cutting out when slowing down.
The first diagnostics machine had difficulty picking anything up on it, but since the new gearbox ECU has been put in it's blocking out diagnostics machines all together.
Here is the list of changed parts. Crankshaft sensor, throttle body sensors, EGR valve, auto gearbox sensor, gearbox ECU, coil pack and idle control valve. All these were second hand and its had a full service. Plugs, leads, filters gearbox oil was also changed. When it's cold it's been fine until it's warmed up then that's when the problem seems to be more active, which only takes 2-5 minutes more recently before the warning light starts coming on then going back off again.
When the car is going to break down but manages to pull through it tends to speed up on it's own, which it also does when started back up after a breakdown.

Thank you in advance.