wiredTiger cache 95% performance issues

Hello,
We are experiencing performance drop off's when the wiredTiger Cache becomes 95% full.

Performing a mongodump on numerous databases serially causes the wiredTiger cache to become 95% populated.

In some instances the mongodump halts and our backups do not complete.

What I think is happening is the cache is growing above 95% used. At this point mongo is trying to clear data from the cache and failing, at which point mongodump hangs.

Changing the size of the wiredTiger cache (up or down) can cause the process to continue, I suspect its causing a forced flush of some cache data.

Please can you provide a list of data you would like to me to capture and a upload location for this.

Kind regards,
Luke