Quick question to the massive!

I have recently moved all of my components off my Santa Cruz Superlight and onto a new Cove Hustler. Some stuff replaced and some not. The only drivetrain item replaced was the front mech which I swapped for a SRAM X7. All of the drivetrain is well worn but works ok and is all the same age except for the rear mech which is probably 5 years plus old. Drivetrain is all SRAM X9 with middleburn chainrings.

When I am on the largest cog at the rear and on the middle front chainring the chain jumps off the middle ring on its own onto the inner chainring. I cannot work out why I have tried shortening the chain and tightening up on the rear mech but still it jumps! and its not even when going over rough ground and the front mech is not moving when this happens it just seem to want to.

Anyone have any ideas what would be causing this or is this just a symptom of my drivetrain needing replacing?

sounds as though your front mech indexing is just out. If you think not, then you may want to try rotating the mech on the seat tube so the cage is toeing out/parrallel with the chainline.

tightening up the front mech cable a little bit should do the trick.