Bike is a relatively new Cannondale Carbon Synapse with the Ultegra 6800 groupset (although front chainrings are FSA). Has about 1000KMs on it.
Bike has been great, but I went out on a ride today and about 40K in or so it started losing the chain off the inner front ring. Prior to this it was tuned perfectly and then suddenly I couldn't pedal more than 10 rotations without the chain coming off (not shifting at all).
I took out my tools and adjusted the L screw to tighten up the gap and it still happened. Then I thought I'd just keep out of the lowest rear gear so I could just get home (I was stuck going up a big hill). Still no luck. Must have tried 12 times and lost the chain every time within 10 feet of starting up again - making adjustments every time. By the end of it both the front and rear derailleur were adjusted to be pulling the chain away from the inside brutally - just trying to get home. Eventually I had to give up and call for a pickup.
The only other thing I noticed was that immediately before this happened the rear cogs were getting funny - it kept wanting to shift down (before the hill - was in a higher gear so in the big ring at the font - didn't lose the chain, but was going all squirrelly in the rear).
I can't see what is doing it. Chain rings don't have any bent teeth. Seems to only really happen under load. I can tighten up the L screw to the point where it is rubbing the chain and it still will manage to come off if I put any load on the drivetrain. I don't even see how that is possible but it is what happens.
I have no idea what is going on - any advice?