This is normal behavior - one common misconception is that once the reward starts going up, it will remain up. This is not true as the agent may start exploring a completely different part of the state space and that can lead to sudden dips in the reward as you can see.
Once you observe good behavior for a few episodes in a row/good average behavior over a number of episodes, that's a good indication that you can stop training. So I would stop training after episode 50/60 in your case and see if the result works, or I would let it train for more time and see if it recovers/surpasses the previous max (this is also common)
