Dang CEL again...

[Boxtaboy]

Ok, a few months ago, while pulling into my office parking garage at a steep incline, my CEL came on. Bought a code reader, and it said the code was P1128. Only one bank. I cleared the code, and it didn't come back for over 500 miles. Thought it was just a fluke.

Then, last week, at the same incline at my work garage, it comes on again. I clear the light again, and then 60 miles later, it comes on again. Just cleared the light again. During all this time, I am not experiencing any strange engine idle or drivability issues. Power seems strong, and car runs great. No smoke at startup from tailpipe or anytime thereafter. Any ideas anyone? Car is at 61k miles and is an 01 Boxster.