I purchase a used 2005 chevy aveo about 6 months ago. The engine was replaced with a used one due to timing belt failure (before i purchase it). Since then, after a five minute ride above 60km's, the engine and hold lights will start flashing. (engine light first). The transmission will also stay in 3rd gear, only after accelerating enough for the overdrive to kick in. Stop, shut the car off and restart, everything is fine, for about another five minutes. Code P0300, random missfire shows up on my OBD2. But the car works perfect, no miss and plenty of power even in 4th gear. Checked mostly all the things i have been reading about on this site. New plugs, checked for air leaks, visually inspected wires and coil, unplugged every sensor i could think off and replugged, Compression on all four cylinders are between 185 psi to 200 psi. Today i had the car into a mechanic's shop and it was hooked up to a Dianostic Machine. The was no problems to be found. He even misted the wires and coil with soapy water just to see if there was a problem there. Since buying the car my daughter has put about 3000 km's on it (to and from school) without any problems or high fuel consumption. If there was a problem, you'd think the cat and oxygen sensors would be destroyed by now. If there was a miss in the car i would do the elimation process and replacing each item until the lights were fixed but it works and drives as good as a new one. Any ideas or reccomendations with this issue are greatly appreciated. Also is there a way i can disconnect the Hold Mode all together, i hate to use it on the highway if it remains in 3rd gear.