It wasn't sloppy investigation, in my opinion he did a pretty good job, it was his denial of ever using the n word and then being proven wrong that hurt the case.
YOur right about his lying on the stand. I would never convict anyone once the prosecution was caught lying about anything - if they lied once, they lied twice.
However, it was also strange what Fuhrman did, why was he the one that found the glove, why was he the one that saw the drop of blood at a great distance, etc. Too many things against OJ were found by the cop that lies on the witness stand.