Frequent Hardware Failures

It could be many things, not just electrical issues that caused it. One likely candidate might be thermal issues? Is the sufficient airflow in that part of the rack - i.e is something blocking/disturbing the airflow? Is some other device that's running hot blowing onto the BIG-IP? Another far less likely possibility might be vibration.

The other possibility - just bad luck. Initial device failed - replacement device may have been damaged in transit just badly enough to boot up the first time, but fail after hitting some threshold (e.g. temp?)