For some time, I've had a problem with the performance of my Ford Focus 1.6 TDCi (Diesel): when accelerating hard in the 3rd or 4th gear, there was a performance drop and I sometimes had to go off the gas pedal for gas to be registered at all again. The drop was much more severe than a turbo lag.
The strange thing is that after the temperature dropped one night by about 10°C (and has stayed at that level), the problem does no longer exist. I'm completely at a loss as to what could have been the problem. (The lambda sensor should not be sensitive to 10°C changes in ambient temperature, and I don't know what else would be.)