You can try to increase the tension by shortening the chain or adjusting the b-screw. But what also may be happening is the derailleur is not properly aligned. Normally this doesn't show up, but when the chain rides a tiny bit it tends to shift, another idea is their is ice inside the derailleur in this case you can't do much unless you've got a friction shifter option to trim the derailleur.

Apply a lot of chain lube also.