I'm thinking it almost has to be a network issue. You mentioned you were using HP Blades. Is the HP switch to which you referred integrated into the blade chassis? It would be worth a call to them if you haven't done so already.
How long have you been running ESXi 5 on these blades? Was it running for a while and then started to have issues? Any updates applied lately? Have you looked for networking driver fixes?
Did your storage stay connected during these issues?