How to lower difficulty on blockchain? I created a coin (hexco.in)

The problem now is, sometimes some user create pools and use rented mining rigs to mine on my coin. But after a while they stop and my blockchain stucks at a certain block. 

My question now is, how can I solve this problem? I only have a homserver and a raspi2 running as mining service 24/7 for my coin and a seednode running 24/7.
How to lower difficulty on blockchain?


  The problem now is, sometimes some user create pools and use rented mining rigs to mine on my coin. But after a while they stop and my blockchain stucks at a certain block. 


You could include in your expected difficulty calculation a factor of time, where say, for example, every 10 minutes since expected next block time (supposing you have a ten minute average block time) the expected difficulty is lowered by a factor, say divided by 2.

The problem with this method is that it may allow cheating unless you also carefully control network time. If it is not carefully managed then a cheat could submit a block with a future time and a lower difficulty.