There are 7 versions of the xbox on sale; even if you buy a brand new one, it still often can be a version 1.0 release model, due to old stock (I recently saw some 1.0s on sale in a major retailer). The different xbox's on sale not only have different DVD drives (the Thomson drive is renowned for giving DDEs). They also have different video chips in them (3 diferent types to date, I think).
I'm not 100% sure, as I am not a games developer, but I think the differences between the components of the different xbox's have an effect on certain games, leading to some people saying that they have flicker/slowdown/jaggies, etc.., whilst others say that they don't. The responsibility for the problems in certain games is entirely down to the developer (no names mentioned), as these bugs on certain xbox's are down to a lack of thorough beta testing across all 7 versions. I hope this has come some way to explaining many peoples problems.
Changing scart cables will not really help in the majority of cases; the difference between the cheaper cables and the more expensive is two fold. 1)The expensive ones are shielded, 2)They also have gold plated connectors. Shielded cable is only necessary if you live under a train line or somewhere else where there is massive amounts of outside interference. You will only see the difference that a gold plated connector makes if you have a $2000 TV and have perfect 20/20 vision. Don't be fooled into paying money for something that isn't really worth it.