Can someone advise please? When I am using Rapture, I don't seem to be able to maintain the volume which keeps rising and fallin going from zero to high.

It's obviously something I'm doing or not doing but it doesn't happen with any other soft synth

Please help as I found a great Digi Grand Piano sound but the fluctuatng volume is driving me crackers

Cheers Colin Brock

great Digi Grand Piano

 
Try a different instrument see if the same thing happens.  It is probably the "great Digi Grand Piano " you found. 

If it continues to happen, probably a silly question, but you're not using the demo version of Rapture are you? if you are then what you describe is perfectly normal - the demo version of Rapture fades the volume away and brings it back up again every 30 seconds.