I got a new bike a three or four weeks ago and have noticed the issue arise this last week or two.
When I'm going up a hill (or any heavy load), my gears will slip. I don't think it's moving gears, it just seems to slip a bit. My pedals will lose tension as I hear it slip, then will regain traction about a half a crank later.
This happens pretty regularly while pedaling in low gear, maybe once or twice a minute.