This is most probably caused by an issue with your telephone line.
Sometimes we have to slow connections down in order to solve dropouts or connection problems. Also, you may find that there is noise of some sort on your line that could be affecting the modem.
Some modems, particularly the more expensive brands have a feature called retraining. This means that if there is any interference in the line, then the modem will automatically reduce its connection speed to compensate and maintain the connection. If you connect at a higher speed, your modem may be renegotiating the connection and dropping the speed below even 33.6.
This issue could also be caused bv the modem itself. I would recommend looking at your modem manufacturers website and getting the latest drivers and firmware to update both.