Low power engine while driving

This concerns a 2007 Jeep Liberty, well maintained, 65k miles.
3 episodes over 10 days where the vehicle loses most of its power while driving.

I get warning signals on the dashboard (number 26 and number 28. Page 166 in the manual). 26 is electronic throttle control warning light; number 28 is electronic stability program for traction control system.

Each time the jeep lost power, would only do about 15 or 20 mph, I pulled over, I turned the engine on and off a few times, and the Jeep would start up and run fine.

The first 2 episodes happened on the same morning, and I drove over to the Chrysler Jeep place and talked to a couple guys in the service department. I described what had happened and they told me they could not do anything to find the cause because the warning lights were not on right now and a diagnostics test would not show anything -- the error codes do not get stored.

I bought an OPD2 reader to use while the warning lights were on. This morning it happened again (3rd time). I pulled over, with the engine running I plugged the thing in and got no codes. I shut the engine off, then keyed to "on", and again could get no codes from the OPD2.

Does anybody have any idea what is going on, and how to fix it?

Many thnx