Nick,

It sounds as if there may be more than one issue as you mention that in lighter breeze it goes fine but once the breeze comes in....
Diamond tension will be a start...and these are an issue of their own as preference says whether to run them tight, firm or .....but worth a check to see where they sit in relation to the mast track (rake)...again no hard and fast rules here either
Look at rotation position, jib sheeting positions , jib luff tension and then mast rake....but one at a time and remember to keep a diary of what you have changed and from what settings to what settings, wind conditions, sea etc etc....if it does not make a improvement, change it back and on to the next issue....there is no hard and fast rule to make them go fast.....we spent most of last season sorting youngest's boat out and thnk we got it close to season close...

Enjoy the challenge...always happy to help.

David
"Banh Sidhe"