taterball

Hello, The history report: I just replaced the engine with a GM crate motor and had the tranny rebuilt. It had 257000 miles on it and was using a quart of oil in less than a 1000 miles. It had no codes before, and ran good, just used A LOT of oil. The engine swap, got a thorough going over, with the lower and upper intake getting cleaned, exhaust cleaned, throttle body cleaned, and everything else getting spanking clean. I also put in the upgraded injectors. Plugs are Delco new, rotor cap new since the old was broken on install, and the rotor button, distributor, coil, and spark plug wires where replaced about two months ago and maybe 1500 miles on them, and the Mass air flow was also spanking cleaned.
When taking it for its initial test ride, after breaking it in for the 10 minutes it spoke of in the instructions, the check engine light starts blinking at around 65 mph, and when the speed goes below 40 for about 5 seconds it goes off, and stays off until you get it up to about 65 again and it goes to blinking, and goes off when you reduce the speed below 40 miles an hour. There is no vibration or rough running going on at all. I checked vacuum and it is steady at idle and sprayed around the lower and upper intake with no change in rpm. When I get it back to the garage, the check engine light is off, and when I check the OBD it has a P0300 code.
Any Ideas as to what the problem is and how to fix it. On chevy forums I have read several articles and they state that it can be plugs, wires, cap, coil, or catalytic converter. I believe that I have ruled out everything except the catalytic converter and am replacing it tomorrow since it also has 257000 miles on it. Any constructive help will be greatly appreciated.