Erratic quality scanning and verification

I have a DH-20A4H IDE drive, and one thing I’ve been wondering about is the quality scanning that I do with discspeed. If I select a scan speed of anything over 8x, it will either not scan at all, or start to scan, but stop half way through, and keep spinning the disc, but the scan has stopped. As long as I scan at 8x, it seems to be fine. Is this normal? My old Lite-On could scan evan when set at 16x or maximum. Also, if I run a second scan immediately after, I notice the scan can differ quite a bit.

Also, when I verify burns in Nero or Imageburn, the verification process is extremely slow, and often times slow down or even stop briefly at times throughout the verify process. Should any of this be a concern?