Agreed it won't be the cold start valve. The warm up regulator is a possibility as Flipman says and fuel pressure tests (system, control and residual) are definitely worth doing. Also worth checking for vacuum leaks.
Other possibilities include:
Idle control valve (if your car has one - small cylinder with electrical connection, attached to the rubber boot near the throttle body). These get gunked up over time and cause idle problems. Very easy to remove, spray carb cleaner in and watch all the muck come out!
Another possibility is debris in the fuel tank building up somewhere (e.g. fuel filter) and causing a restriction, however I think this would be likely to become obvious when there were higher fuel demands from the engine rather than at idle.
Could also be an electrical issue such as a bad connection on the coil, hall sender (connector on the distributor), or ignition control module. These type of issues often only become apparent when the car gets hot.
Incidentally, are your new dizzy cap and rotor Audi or Bosch parts? I recently had an issue with a new rotor arm on my Coupe, after only a couple of hundred miles the rotor (which was aftermarket - JP Group) starting losing spark through the bakelite to the distributor shaft. Took me ages to work out what the problem was! The symptoms were different (hesitating and misfiring which got progressively worse until it conked out completely and wouldn't restart), but worth knowing as the last thing we all suspect is often new parts!
Keep us posted!
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