Occasional slow down of hard disk I/O

I'm exeperiencing occasional slow downs in my hard drive array I/O running ESXi 5. I'm using Dell R510 with PERC H700 controllers with near-line SAS drives. When all is good, I get about 180 MB/s. However, at times this drops down to 50 MB/s read and under 10 MB/s for writes. I've yet to find a way to repoduce the problem, and it will go away as abruptly as it starts. I only have 1 powered on VM that uses this array right now. I've powered off or moved the others to try to find the problem.

Any suggestions are welcome and appreciated.

Thanks,

George