I friggin' hate rubber band AI and I'm starting to be pretty sure that GST has it.  I can't tell you the number of matches I've played in which I've won the first 2 games and then all of a sudden a lot of my shots start going out of bounds and everything falls apart.  I then go on to lose the match.

THat's the worst.
Fifa has that too.

I seriously doubt I'm the only person this happens to.

In Fifa, on the god level, the only way to win is to pass a lot because the opponent never gets called fouls. However, even on a pass, they literally grab the ball out of the air with their hands and pull it down.

It's just cheap programming, I stop buying those titles once I discover it.

I am willing to grit my teeth and accept it on higher difficulties such as in Mario Kart Wii, however on easy mode this shouldn't happen. I swear to god I'm not doing my shots any differently in the later games than I was in the earlier ones that I won. Does it have something to do with my player's stamina? Is he getting tired? Will things such as my shot accuracy improve as my player levels up?

Or you just break under the pressure of almost being able to win a match haha.

But yea lots of games have it (especially racers) so unfortunately it is a common annoyance.