I upgraded an old 515 from 5.2 to 6.35. Internal hosts where then not able to perform DNS.
Also, internal hosts were intermittently having reachability issues.
Also, when performing a show xlate, some global / local address where correct and then some global / local pairs where screwed up such that the internal(private) IP was shown as the global and the external(public) IP was shown as the local.
When I copied over the 5.2 configuration I noticed an "alias" line. Could this be causing the problems?