Are you using the latest firmware for your burner?
Take a look at Plextor's website for your region . . .

The PX-750A only has a 2mb buffer. I wouldn't recommend attempting on the fly burning with this drive, because there's a good chance you will engaging this (and doing so, generally, doesn't create the greatest burns):

"Buffer underrun proof

The buffer underrun proof technologies offer a solution to prevent writing failures during the burning process for CD and DVD by pausing the burn if the data to the drive write buffer is slowed or stopped. The drive resumes the burn process after the drive write buffer is refilled."

Pausing the burn does not create great quality backups, imo. In general, I really try to avoid making my burner use this feature.

Also if your reader happens to be on the same channel as your burner--or if your reader is a slave to any other drive, then burning on the fly is also not the best idea.

The last good (real) Plextor drive that contained a decent sized buffer was the PX-716 (8 mb). The PX-750 is really a Teac DV-W516E or a Benq DQ60 drive (based on the Panasonic or Matsushita chipset--as opposed, to in my opinion, the better Philips chipset based burners sold by Benq).

Lastly, I would be sending a Clonecd log and asking for some help from Slysoft's email support.