What would cause indexing like this

After upgrading to 2.3.3 I started to see my indexing in marvel look like the below:

What would cause this. I am use to seeing a smooth graph with some spikes but nothing like this. It occurring around update to 2.3.3 may be just a coincidence.

I'm not sure what to post to help with this but my set up is:
12 data nodes with ES 2.3.3 24 GB RAM (upgrading to 32 tomorrow)
12 logstash nodes with LS 2.3.3 and ES client nodes running ES 2.3.3. LS pushes its data to these client nodes. 16 GB RAM (upgrading to 32 soon)
5 shards 1 replica per index.
LOTS of storage
I am ingesting data from many bro sensors


It's not GC is it?

it was. Thank you. Fixed.