I don't trust Consumer Reports, Raod & Track. etc... when it comes to car reliability and purchasing suggestions. I personally have found many of the results close minded and inacurate. They tend to lean heavy in favor of imports, no matter what the makes and models they are comparing.
Relabilty has a lot to do with the owner and not all the car (just to clarify, some cars may be more reliable than others. Read further before bashing this comment
). Who are the most likely owners of a Sunfire GT or a Grand Am SE? Chances are a teen will be driving it and will drive it more aggresively causing it to wear and weather faster. Also a teen is less likely to keep up on scheduled maintaince. For these reasons the long time durabbility of some makes and models can be scewed by the types of drivers it attracks. Don't get me wrong, some cars are mechaincal nightmares , but there are to many variables to trust simple statitcs that they usually come up with.
If they could go into detail (which you said they didn't) that would help alot as to determine the nature of the lower rateing. I would agree that newer or newly redisgined cars inherintly have more "flaws". The first 2-3 years of any new car is a transition phase where they are working out the bugs. For the 97 Bonnevilles it had recently moved to a full OBD II platform but the body was relatively similar to previous models. 2000+ was a whole new car. I'm sure alot of "new cars" get poorer ratings the first few years for the above mentioned reasons. Besides, if all the cars got good rateings, what would you need them for for? That would be bad business for them.
If you want the good scoop on how reliable a car is and how well it performs, the true test is to talk with those individuals who have been driving them day in and day out for some time to get their opinions. If it is a new car, buy it and try it. No easy way around that.
(ok, whoever I stole the soap box from can have it back now, thanx)
Sorry if I strayed. I've just found that too many times people embelish the facts with opinions and here say. I have been burned a few times on trusting what publications state a product can or can't do. I prefer to do a little leg work myself now when makeing any substantial purchases, including cars. Too many people out there too willing to tell you what you want to hear vs what the real story of it is. Just think, Consumer Reports (and other places liek them) are businesses. They will do and say what they need to to keep the $$ rolling in.
(ok, here'* your soapbox, for real this time...
As for the initial question... The years you mentioned don't have any 1 factor that really stands out as a contributing cause to a poorer rateing except for the 2000+, and that is just because of the redesign.