Pyes indexing problem

Hi All,

I am currently evaluating ES as an alternative to SolR.
My test environment consists of 4 m1.xlarge instances on EC2, 8GB (out of 16GB) is allocated for ES JVM.
The index is broken into 5 shards with 1 replica.
For indexing JSON docs I'm using "Pyes" with "Bulking" enabled (chunks of 100 docs).

I've encountered a problem when indexing about ~1.5 million of docs (same index) where I get time-out from ES and unable to index any new documents .
The system load is normal, no swapping accrued, I also I couldn't find anything relevant in ES logs.

Has anyone faced similar problem?