Atenolol pills seem to vary on actual dosage

Has anyone had the experience of taking beta blockers, with some success, but then finding that sometimes it's like maybe you get a bad pill that has 2x or 10x the dosage it should?

I was prescribed atenolol OMG about 25 years ago for a combination of blood pressure and some minor and generally unspecified heart arrhythmias. They seemed to help. Took them for many years, no real problem. And a tiny dose! I was prescribed a low dose, 25mg twice a day - but when I took the very first 25mg tablet it felt like I'd been hit by a brick, my heart rhythm no longer responded to exercise - so I got permission to cut it back to half (an already tiny) 25mg tablet, per day! And that worked well for me. After some years I raised it to half a tablet twice a day.

But a couple of years ago I had an experience that it seemed to slow my pulse rate TOO much. I started breaking the tablets into even smaller pieces and that seemed to help. Going without, did not seem effective, so it did not seem it was my own metabolic baseline that hand changed. I thought about talking to pharmacist or doctor - but did neither.

Anyway, that passed, and didn't have the issue again - until today. And maybe more strongly than ever. Pulse rate a little low but super-resistant to change by exertion. Could this be dangerous? Well, if the pill variation was maybe 2x the proper dosage it should be mostly harmless, but if it's much greater than that I don't know.

So I'm just asking if anyone might have had any similar experiences or any suggestions under the circumstances. Thanks.