My neighbour has a possibly similar problem in that during the late evening,night and early mornings she receives around 14 more channels. During the day she will get the service is not available or the signal is to weak.
Someone on another forum explained the reason behind this and it involved temperature inversion and moisture in the atmosphere but it went way over my head. But essentially it seems in the summer months channels are limited but during the winter months she should have no problems. As to whether to replace the aerial, jury is still out as there was a hell of a lot of rubbish published about aerials prior to the digital change over.