My check engine light starts to flash if I am on the highway for more than 2 minutes in 5th gear at 3200-3500 RPM and at 83-88MPH. It does not do this in 4th gear at the same RPM. Anyone have an idea as to why this occurs?
A flashing SES indicates a major misfire condition. You'll want to get your hands on a scanner (AutoTap, etc.) to see which cylinder(s) is/are misfiring. It could be your wires, plugs, or a fuel issue.