As a chemist, this sounds like your source of Sinemet is contaminated with impurities. I know in the US your drugs are very expensive, and maybe lower quality drugs get into the supply chain. Yes drugs are tested, but not all impurities show up on the tests used.
My Sinemet is not crumbly, it is well shaped and doesn't give me anything like the problems you describe. I take a dose, and nothing happens for about half an hour. Then I get a tingling in the legs, which signals it is starting to work. Two to 3 hours of on time follows, and in the 2nd half dyskinesia starts and lasts until the end of dose. . As a last resort, I could post you a blister pack of our Sinemet. However, we need to know if any of the other US members have a source that does not give these bad side effects. In which case you could simply switch to them.
I can only suggest you try different sources of Sinemet, branded instead of generic preferably.
Good luck
Ron
PS As antherr idea, why not try Mucuna Pruriens, that works for me too. It contains levodopa.