l-carnitine, NADH and calcium pyruvate all gave me a really nice boost in energy and stamina - l-carnitine helped for a week, I could function without crashing, this was in 2002. And then it never had that effect again.
NADH gave me a really good boost for about 10 days - this was in 2004, and then it stopped, and never happened again.
And calcium pyruvate helped a lot for 3 or 4 days back in 2005, and then it stopped.
I continue to take a carnitine supplement (Carnitall) because I've read how important carnitine.
I have tried dozens of different things over the last 20 years, most didn't do much, but a few have made a noticeable positive difference for me and I continue to take them: folate and methylcobalamin; P-5-P; benfotiamine; andrographis; branched chain amino acids (cut PEM recovery time in half); d-ribose.
For many years I had severe digestive issues which affected everything, and a liver detox helped a lot, plus I started taking betaine HCL with pepsin and my digestion has been good for quite awhile now. I also drink kefir periodically.
I also had a lot of trouble with toxins for many years, which would leave me feeling sick and weak and screw up my digestion. I tried Cutler's protocol, it didn't do anything for me that I could tell. Then I inadvertently came across l-glycine, inositol and glutamine. It's a long story but the upshot is I don't have detox symptoms any more. I think they got my detox pathways working properly.
So there were several different things going on making me feel like crap which seem to have resolved, a lot through trial and error; also muscle testing was invaluable in helping me navigate all of this.
I still crash regularly; my energy envelope has not expanded, but when I'm not crashed, in general I feel much better than I used to. I still do get sick with depressing frequency and regularity, every time I crash, though with andrographis I recover more quickly, and have recently added cordyceps and astragalus. I'm going to experiment with reduced glutathione, which I have never given a fair go, to see if it will help.