I have noticed in the past that when driving right around 50 km/ hr - 30 mi/hr at steady speed, I would sometimes get a very tiny speed decrease then resumption back to normal. The change was small, perhaps equivalent to 1-2 km/hr. At first I thought it might have been a brief misfire, perhaps a coil problem; but, no misfire codes and no sign of coil problems. The problem was very infrequent and in the absence of any evidence of misfires I attributed it to throttle input fluctuations - me moving my foot around on the throttle or bumpy roads causing throttle inputs. When I lifted my foot off the throttle and then reapplied pressure the problem would always completely disappear and not repeat which is definitely not consistent with a misfire problem.

Today I was out driving and the same thing occurred, except this time traffic was light and I had a good distance over which I could try and drive at a steady 50 km/hr. What I noticed is that the car would slow down by 1-2 km/hr for about 2 seconds and then speed back up to 50 km/hr and then about 5 - 10 seconds would slow down again and then speed up. This repeated about 4 - 5 times with consistent timing before I finally had to end the test because of a traffic light. When the light changed I pulled away and drove under the same conditions to see if the problem would repeat itself; but, it did not return. When the problem occurred I was very conscious about holding my foot steady. The speed change was very regular and definite, very much like when the cruise control engages.

When this occurred today the cruise control switch was on; but, the cruise control was not engaged. It had been set for 110 km/hr so clearly it was not attempting to control to that set point and as soon as I let up on the throttle or if I pressed the throttle the throttle responded as it should. At this point I have not determined whether this occurs only when the cruise control is switched on or whether it is unrelated to the operation of the cruise control. If it is unrelated to the cruise control I am wondering if the there might be a dirty spot / dead spot on one of the drive by wire throttle position sensors causing a input fluctuation even if the throttle pedal isn't moving. However, that would not really jive with the consistent speed change repetition frequency when the problem does occur. I have also only noticed the problem around 50 km/hr. It might be present at other speeds; but, I have not noticed it. Speed control with cruise control at highway speeds is smooth with no obvious fluctuations. There are no DTCs stored in the ECU associated with the DBW control.

Has anybody experienced this kind of behaviour and if they have, determined what the problem was? The car is a 2000 model year North American market version.