MDMA is not known in nature, it is semi-synthetic since it can be synthesized from safrole which is found in nature. I also have to disagree with ecstasy tablets containing cocaine or heroin, this is very unlikely as both are impractical adulterants for ecstasy tablets. Just search ecstasydata.org for tablets containing cocaine or heroin, for cocaine there are 5 results and heroin 1 out of ~1,500 pills.

A few months ago I was doing ecstasy just about every other week and noticed the rolls were definately less intense, though increasing the dose may help I don't recommend it. The solution is just to take a break, for a month or maybe longer. With ecstasy (especially) the less you do it the better it is.