My chain keeps climbing off the upper jockey wheel making changing difficult. Mech looks straight, cable adjusted properly, jockey wheel seems OK. When going downhill on anything rough the chain jumps off the middle ring onto the granny ring, again everything seems adjusted correctly. Any suggestions appreciated.

chain too long so not enough tension in the derailleur to keep it on?

I’d check the derailleur top wheel is adjusted correctly relative tot he cassette too – it shoudl be near touching – Parktool has good advice.