I have a muscle issue that has been affecting me for a while (15+ years). As I get older it seems like it's getting worse.

If I ride at an easy pace, I'm OK for relatively long rides (30-50 miles). If I use my muscles with anger - hard efforts, climbing, sprints - my muscles start to lose power quickly. After a short while - minutes sometimes - I get to where I can't do any more efforts at all and on one long ride (70) I had to stop and rest for 15 minutes before I could continue.

I am 50 now and it seems like it gets worse every year.

I also get cramps fairly easily and have to monitor my electrolytes closely, but no amount of mineral eases the power problem.
I also get 'micro cramps' called Fasciculation which don't really hurt but are very odd.

I have larger muscle mass than most cyclists and obviously have lots of fast twitch. I don't lose sprints (amateur .

I have been to the sports doc and got all the tests and they say "you're fine". Part of the doc's take I feel is that I'm older, I don't have any real competitive future and I should just be OK with being healthy. I get it.
But still, I like to race and do OK at least. I like to go on longer rides with friends and not have to have them wait for me. It's a 1st world problem but would love to see if there's a supplement or ______? that I could take that would get me back to something normal.

thanks