Feeling of click/slip when pedalling
You state in the comments that the (pinch) bolts on the left crank had come loose (Ultegra Hollowtech 2). So you’ve solved the problem by tightening the bolts. 

I saw another bike (I think Ultegra cranks) do this yesterday, again it was a very new bike, it was on its first serious outing and the left arm had worked loose. 

I found the question with a relevant Shimano publication extract: How to tighten the Hollowtech II (on R2000 Claris) cranks?

In short, any time you install these bolts, tighten/torque check them again after 20 km, much like checking the wheel nuts on a car, a short time after it’s been in for a service.