Mk3 1.0 MPI loss of throttle response and stalls while driving. Fails to immediately restart.

My Polo Mk3 1.0 MPI petrol from 2001 recently has acquired the habit of losing throttle response with a subsequent stall. This occurs after approx 20 minutes of driving. The car will fail to immediately restart, but will eventually do so after 5 minutes of waiting. After a while, the car will lose throttle response and stall again. The longer this waiting period is, the longer it will take before this occurs. There are no errors on the dash nor has the mechanic found any error codes in the ECU. The car idles fine for hours without any stall, which makes diagnosing the problem harder for a mechanic.

One theory could be an issue with vacuum emerging in the fuel tank, which may indeed explain the symptoms and in particular the waiting period symptom. To test this I drove the car to stall and then immediately opened the fuel cap. The car started immediately after I did so, however I did not notice any "whoosh" of air being let back into the tank, so I'm not 100% sure.

Nonetheless, the fact that the car never restarted immediately after the stall still leads me to think I'm close to the cause.

Before I start replacing purge valves etc, I would be grateful for advice on this problem. Has anyone else experienced this? Can it be expected not to see any error codes or is this unusual? Any advice would be really great.

Thank you.