Is that drive in an external enclosure? What write speed did you select?
The drive probably decided to limit the speed part way because it wasn't happy with the writing quality, or possibly to avoid buffer underruns if the incoming data flow was insufficient.
Use the 'Create test disc' function and post a copy of the result. This will write a disc and record the actual write speed & buffer level throughout the writing process. If the drive has limited the speed again, scan the disc and see if the point where the drive intervened correlates with an increase in errors.
(If you don't want to waste a disc, CD-DVD Speed can perform the same write test using an ISO image file as the source, rather than writing random data.)