This happened to me the other week as well. I was heading eastbound on the M4 by Slough with:
[*] Eco mode enabled
[*] Cruise control at 65mpg
My passenger was asleep, I was listening to the radio, and I can be sure I didn't knock the gear selector. The car suddenly started to slow down, and pressing the accelerator did nothing. My speed hovered for a while at 40mph, but then dropped again. By this time I'd pulled off the M4 and was able to come to a stop on the hard shoulder of the slip road.
As I was losing power, the car beeped at me, but there no lights came on on the dashboard. I think three times it did that beep-beep you get when you've done something wrong.
Once I'd stopped I was able to get going again, and it's been fine since.
I took the car into the garage and the mechanic found nothing in the diagnostic log. However, he helpfully said that his colleague had been on a course the previous week and the instructor had mentioned something about eco mode+cruise control+inclines, and that he'd try to find out what was said.
From my one experience I consider this a software bug - although the battery was drained, there was plenty of fuel, and before I joined the M4 again I made sure that the ICE started OK. I won't enable eco mode again, and I hope that Mitsubishi can find and patch a flaw. My experience wasn't too bad as I was in the left hand lane already, but had I been in heavy traffic, it could have been more serious.
If you've experienced this sort of trouble, would you say that:
[*] Eco mode was on
[*] Cruise control was on
[*] You were on a slope going up
buconne