My daughter's 2009 13" MacBook Pro could no longer reliably access the hard drive to boot properly. A new hard drive did not solve the problem, however the laptop would boot easily from an external drive.
I thought it was the motherboard and replaced her laptop while her old one sat in a corner. When two of her college age friends, who had bought the same model at the same time, had similar issues I did more googling and found this model notorious for the board drive cable going bad.
Buy the part and fix it. If you're having performance problems and you do disk maintenance, it may be the cable. Why wait until it won't boot?