When I start up my autoload - so there no audio or midi recorded and no automation data, and I'm just working out some chords with the Yamaha Grand for instance in the exs 24, the fader suddenly drops to -15db, it happens randomnly and is very annoying especially if I'm in the middle of a take. It only happens on the audio instruments like EVB3 etc and not the audio.

Can anyone help?

What kind of midi input device are you using-does it have any "Fader" controls on it? Perhaps one of the controls is dirty and is sending a midi volume message to Logic whenever it feels like it.

It an M-Audio Keystation 49E.

I might take a can of air-duster to it and see if that makes a difference.

Any other ideas?

You could go to song settings in the file menu and block everything other than note on messages in the MIDI filter. That way at least you will know whether the message is a fader controller created by the Key station. Sort of narrows down your problem. If that doesn't make a difference try the 'default' autoload song. Temporarily remove your autoload song from the song templates folder in the application support folder and start up logic. If the error is gone it was something inside your template.