Problem with automatic transmission

Intermittently the transmission does not change into a higher gear when accelerating (slowly or faster). It is as though it is stuck in first gear although the OD button has not been pushed. It may stay like that for up to four minutes or so and then suddenly it changes. It can happen at any time of day, whether or not the car has been driven (cold or at normal running temperatures). Then for days it may work perfectly normal. The dealers mechanics could find nothing wrong and reset the computer. Hope you can advice.

kind regards
Gerald Veurink