Greetings mactommy - it sounds to me like you may also be dealing with a cleanliness issue. If you have never experienced this issue in the past, now suddenly the issue has come up, perhaps you are becoming complacent in your sanitizing practice.
My priming practices is very similar to what Oginme has outlined. However, I add my priming sugar a little differently. With priming sugar in mind, yeast will gobble up sugar as long as there is sugar available to consume. So making sure the bottles are completely sanitized before filling will ensure there is no additional sugar available for the yeast to consume once it has fully consumed the priming sugar. Again, blending the priming sugar very well with the beer and bottling in very sanitized bottles with help narrow down your foaming issue.
I hope this helps!!