If I had to guess I would say it's westhash. I can't say for sure but I've also been having trouble with rejects lately but intermittently, especially on one machine. I'm not too far below you.
You can view what kind of rejects you have. If it is stale share, then it is probably network problem - higher latencies. If it is any other type of rejected shares - then check your miners/software as something is must be wrong on your side.