There are toner streaks in the printed output running in the direction of the paper feed. (CX1200)

This is caused by a bubble, bump or wave in the paper that occurs between the transfer of the toner to the paper and the fusing of the toner to the paper.  Before the toner is fused the paper rubs against the fuser intake causing the streaks.  The wave is caused by the transfer motor and the fuser motor running at different speeds.  They need to be close to equal otherwise one motor is pushing on the paper faster than the other motor can pull it.  This is normally factory calibrated.  However, this setting may need adjustment over time especially if different media is being used or the fuser was recently replaced.

Instructions for Increasing Fuser Motor Speed:

1. Go to the secret diagnostic menu.  See instructions below.

2. Select Printer Setup.

3. Select Engine Setting 1.

4. Use the right arrow button to increase the speed.  Press the check to set the value.  Increase the speed in small increments.  Increasing it too much will still fix the problem but will also introduce new problems.

Important Note: Please change nothing else in this menu.

5. Switch off the printer and turn it back on. 

6. Print again.  If the problem is still present, repeat these instructions, increasing the speed each time until the problem disappears.

Final Note: Toner dust and debris in the paper path can also cause a streaking effect.  See this article for some maintenance tips which may solve this problem.