Might be chain suck.

Have you fitted a new chain recently ? How much use mileage has the big chainring had ? Might be time for a new one.

Some good info here
J LEVY | BikeTechArticles | Chain-Suck : Its causes and mechanisms, How to avoid, prevent, and remedy
Diary of a Mountain Biker

Also check there are no stiff links in your chain.

Chain lube will not make a differrence.