1. Your data source can't keep up with the CD-R. Try using disc-at-once writing from a disc image with the speed set to 1x. If it seems to be getting worse over time, you may just need to defragment your hard drive.
Be sure that there aren't environmental factors creating difficulties. CD-R units are usually built to handle small shocks, but having a set of speakers playing loud music on the same table as a CD-R may cause it to skip, resulting in a failed write. Sonic booms, heavy construction equipment, and nuclear detonations may have similar effects.
2. You have a bad batch of media. Try a different type and brand of disc. Some distributors (e.g. dataDisc) will exchange media that's provably defective.
Be careful with Advanced Power Management functions on some PCs. If the keyboard and IDE devices are completely idle, the system may decide that nothing is going on and switch to a low-power mode. Ditto for screen savers that kick in after the system has been idle for a certain period.