Thanks...I really don't know why I'm collapsing after being ahead, sometimes by large margins.
Therein lies your first step. Before you can fix anything, you have to know what is and what is not working. If you don't know that, you can easily break something that was working. I agree with Scott, see a qualified instructor. When you have a solid PSR, you don't have slumps.
That said, there is a little you can do now. Try and identify if it is a mental problem or not. Are you getting cocky when ahead and letting up a little and getting sloppy without really realizing it? Are you so convinced now that you will lose that your mind is finding ways for you to lose? Your mind will try it's best to do what you tell it to. If you are thinking of losing, it will find a way to do just that.