Erratic Hesitation

My pd 170 vrs 2007 has started to hesitate and is really jumpy ,but it is only happening when the engine is cold and the car is sitting in the sun .

when the weather is cold it dosent happen ,its only on warm days, after its been running for a while its fine.

Also there is sometimes a musty smell coming through the vents when the blowers on but this also only happens no and again.any ideas people.