I have seen this problem before I think. The RA (or Dec) guide corrections may have become inverted somehow (There may be a setting somewhere) so instead of correcting towards the right position it corrects in the opposite direction and the star continues to drift at an accelerating rate away from the guide position. It is connected with the meridian flip I believe. If you train and then guide on the same star does it work correctly ? Does it then not work if you move to a different star on the other side of the meridian ?
Robin,
A setting in PHD2 or TheSkyX?
I will need a clear night to experiment with this.
Jack