Peugeot 307 misfire/poor running warm up phase

initial start up is fine,whilst exhaust gas
recirculation pump runs for +,- 20secs engine
still runs fine. As soon as pump stops the
engine starts to shake and develops a misfire
a very strong sulphur smell emits from exhaust.
The vehicle cannot be driven at this time.
after approx 2mins the problem goes away and
will not return until engine temp is cold.
No faults show up on diagnostic test!
I have replaced map sensor and have removed
cat from exhaust.
Any ideas? please reply

Thank You.