Resolved Cumulative Reward Graph goes up and down

I keep getting the cumulative reward graphic with the shape of a mountain, like it learns how to do and the rewards goes up but in the end it, the reward start to go down as you can see in the image. What should I change? Why is doing this?

 

After reading a little more I did make the Reward values smaller and also, as I read that the batch size for discrete value is better to have it around 512; and for these runs I did use a batch size of 1024, that could the problem.

By changing the reward thing and the batch size I am now getting stable graphs.