Solved

Winning by Grand Slam and getting -1 reputation increase

Just curious if my game is robbing me of reputation for at least the last 3 races I've won by Grand Slam and got -1 reputaton Increase 

It's because you are in inactive league. If you won't change your league this will continue. Why to increase your reputation if your are the only active player there?

I'll echo what Hjbjg said, however, the core reason is because the managers you are racing among have far too low reputation. I have the same problem in my league, I have the most reputation there, however, since I am constantly winning among those with lower reputation than myself, it means I need to search for tougher competition if I am to increase it.