Timestamp and _timestamp

Hi all,
I have a ES cluster with four nodes and 157 indexes. There are about 140
mln. entries that occupy around 50 GB size (1 primary index with one
replica). There are 2 data nodes, one pure master and one client node that
serve as gate for web requests.
Last days I started to observe that the cluster becomes very unstable and
every few hours one of the data server stop unexpectedly. The only solution
was to reboot all data nodes to be able to process future logging.
My mapping contains this definitions:
"Timestamp": {
"type": "date",
"format": "date_time"
"_timestamp" : { "enabled" : true, "path" : "Timestamp" },

After some tests I discovered that if I do request with filtering on
Timestamp the CPU load becomes very high and the cluster gets unstable. All
incoming events are rejected.
While when I make requests filtering on _timestamp everything works well as

My question is: why this is happening and what is the source of this
Any ideas how to fix it?

Thanks in advance,
Nickolay Kolev

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