Misfire cyl 6
 

Recently purchased an 03 DII with 62k. Service engine light was on and blinking. Had loss of power. Felt like misfire. Got codes read, and it was reading 10 codes, all misfire codes. Replaced all the plugs and wires (thanks to DeepBlueWater's post). Reset light. Previous owner had started putting regular gas in it. I put premium back in it with a can of Seafoam and also cleaned out the throttle body. Ran good for a day on a two hour drive.

Now it is losing power again, but only between 35-40 mph on inclines and light blinks. At this point the truck begans to shake a little like its struggling for power. Once I'm over 40mph it's fine and can cruise at 70mph with no blinking light. Feels like this only starts after it warms up. When stopped it has a little bit of a rough idle. Only code is misfire in cyl 6.

Any thoughts on the problem would be much appreciated. Thanks.

Stop driving it while you are trying to decide what is wrong or you'll eat a cat.
Get it read again, what were the original 10 codes, what is showing up now? What brand of plug and wires did you install?

The only code showing now is the P0306 (twice), misfire in cylinder 6. The original codes were P1300, P0300, P0306, P0301, P0307, P0305, P1415, P1412.

The P1415 and P1412 never appeared again after I reset the codes. The only one that repeatedly shows up is P0306. I used AutoLite Platinum plugs, and 7mm inexpensive wires. I know I should have used Magnecore after reading all of these posts, but I was trying to do a quick fix to see if this was the root of the problem. I don't think a plug or wire should go bad that quick. Could it possibly be a bad plug to begin with and have a bad gap? I will pull the plug tomorrow and measure the gap.

Thanks for the help.