There must not necessarily be a reason, such as a sudden market change. There is also the possibility that the performance gap is just a random fluctuation of the distribution of your trade results.Quote from dom993:
I developed a 1st version that I took live mid-September 2011, only to stop trading it early November as the system experienced a severe loss of performance (which really had started early August). That loss of performance did continue into the new year, until around the start of active trading of CLH12 (late January). Meanwhile, I was frantically scrutinizing the charts, looking for clues. But really, I couldn't find any "reason" for that loss of performance, aside from using "bad" pivots.
Finding out if it's a market change or a fluctuation is relatively simple, by generating a set of Monte Carlo resamples of your equity curve, and checking the resulting curves for similar performance gaps. Here's a link to a paper: http://www.algodude.com/monte-carlo-resampling-of-equity-curves-using-n-bar-segments/
However, from what you describe, I suspect that the gap is indeed caused by a market change and will likely continue. You're trading price action, which means your results depend on very temporary and volatile market properties. One could also say that price action is just trading noise. This does not mean that your strategy can not be profitable, but it might be short lived, especially since you said that you manually tweaked the strategy parameters a lot.
For making such a strategy stable, you could formulate an optimization algorithm for those parameters, and then do a WFO optimization over a lot of assets and time periods. This could give you at least some good information if you should continue to develop and trade it, or give it up.

