Low Oil Pressure Light

The head gasket blew on my car about a month ago and I had it repaired for the second time (it previously went about eighteen months ago). After having it repaired, when driving the car started displaying an 'Oil Pressure Low' light on the radio screen. This usually occurs when you have been driving for over half an hour and are stopped in traffic. If you turn the engine off and on again it seems to disappear. I have taken it to the garage since and they cannot find anything wrong with the car. They have changed the oil switch, but this has not helped.
Any suggestions as to what the problem is would be very welcome!