Two red lights on Xbox 360 after I have fixed initial 3 rings?

I just fixed an Xbox 360 using the as5 Thermal Paste, M5 80 screws and washers. It turned on fine, the RROD was gone, ran a game no problem. So I decided to sell it, ends up, after a few minutes the guy who bought it e-mails me saying that it got the 2 red rings, which means its an overheating issue. What did I do wrong and what can I do in the future to prevent this from happening? Use more thermal paste for the heat sinks? Thanks in advance for your help :D