some things to try maybe:
try a lower speed setting.
shut down any other background programs that are running before burning a disc in case they are interfering with the process.
defrag your hard drive.
try different brand cdr media.
try cleaning the lens.
restart your computer if you havnt done so in a while.
im not familiar with roxio but try and increase the buffer size in the software slightly(depending on how much ram you have,set to no more than about a quarter of total memory)