This isn't a job for the home handyman. There are two fuel jets, when the
engine is cold or when you are driving hard both open and gasoline pours
into the engine manifold - just like driving with the choke always open.
When your engine is warm and idling or running along gently, one jet is
used - to save fuel. Either a jet is fouled or someone has been fiddling
with the mix adjustments. A good peugeot service agent with a history,
ought to know how to fix it. A carburettor and injection specialist would
do a nicer job, probably clean and tune the injection assembly as well -
it would be better than new.

Good luck,

Ed.

exactly what my Nova was/is doing.

after spending nearly ?350 on electrical parts and vacuum sensor and garage
fees it turns out to be the ECU.

At cold the engine temp sensor was fluctuating between 30 and 80C thus
altering the timing and fuel injection. The temp sensor was renewed but the
fault was still there. Vauxhall told me it's the ECU which I have now sent
away for checking/repair.

dj

Have you checked the idle stepper motor?

attached to the main throttle air valve.

If this is gubbed then it does what you have described.

Around 40 ukp from eurocarparts.