A Consistency Check does pretty much what it sounds like ... checks the consistency and integrity of the array, fixing corrupt areas where possible. You can't do that now, but if you have been regularly doing them, then I would lean more towards a bad disk or slot (we haven't ruled out the backplane - obviously the entire backplane board is not bad, but the first slot certainly still could be - it is just less likely).
Possibilities, in order of likelihood:
1. Disk 0 is bad ... as unlikely as a "new" (replacement) disk is to be faulty, it is still the most likely scenario.
2. Your array is corrupt ... over time, just like an OS file system, a virtual disk/array can develop corruption. The disk may be going offline when a particular corrupt location is read and cannot be reconciled across the array, causing the controller to think it is the disk's fault. Normally with this though, the rebuild will never complete.
3. Firmware bug/mismatch ... your controller is pretty out of date, and your drive firmware has the potential to have three different revisions. Now the firmware version between brands is not so important as having them all at the latest. Between the differing firmware and out of date firmware on the controller, it is possible that the controller and drive(s) are simply not on the same page with each other - newer/older commands not receiving the expected response, etc.
4. Bad slot ... like I said, slot 0 on the backplane could be faulty, causing the drive to lose its connection and time out. Normally, however, it would be displayed more consistently, causing the disk to behave strangely all the time instead of sparsely random times.
5. Bad cable ... like the bad slot, it is unlikely, because the issues would be more widespread, but if the wire in the cable going to slot 0 is damaged, it could cause the disk to timeout/fall offline.
6. Bad controller ... this is the least likely of all, as issues would not manifest itself on a single drive/slot.
I would test the drives first to rule out (or confirm) that as an issue.