Hi Thomas,
From what you describe in your post, I assume that you scored 680 (and not '608', which is impossible). Before I can offer any advice, I'd like to know a bit more about that performance and your timeline:
1) When did you take that GMAT?
2) What were the Quant and Verbal Scaled Scores on that Test (the numbers from 1-51, not the percentiles)?
3) How much time did you spend studying for that attempt?
4) What is the application deadline for the PhD Program that you're interested in?
GMAT assassins aren't born, they're made,
Rich