Is it the 2.0? Excessive black smoke?Air mass sensor could be one culprit, hot air is less dense but that's only the wildest of guesses
On a similar subject I have a Kia Sportage diesel that keeps slipping in and out of limp mode with no warning light coming on. Have had it plugged in and all faults cleared etc several times, but it still does it. The only pattern I can see is that it's more likely to happen on cold days.