You might try pulling the plugs and inspecting them to see if any of them look different.

I had a similar problem that went on for several months, slowly getting worse but not bad enough to throw a code. It turned out to be a bad fuel injector.

When it finally got bad enough that the truck ran really rough and it threw a misfire code, I was able to use that info and look at the plugs and see that the cylinder indicated had been running lean (HOT). That plug was totally worn and glazed/brown compared to the others that had minimal wear and were just a little bit chalky.
Unfortunately, the collateral damage was worse than just the spark plug: it took out the head gasket too. <img src="/forums/images/graemlins/frown.gif" alt="" />

In hind sight, I probably could have diagnosed the bad injector by looking at the spark plugs long before it got really bad and caused other damage.

Good luck!