Get your eyes tested, quite simple really...
As has been mentioned, especially as we get older our eyes find things more difficult to resolve properly as the lense in the eyeball starts to harden and so is more of an effort (i.e. strain) to focus on things which are slightly out. Also, aging eyes won't be able to compensate for minor eye problems like young fresh ones will!
I have been through many a pair of top shelf bino and came to the conclusion that I was never going to be happy, I can never get them quite 'perfect' no matter what I do with the diopters, although the key to 'almost' perfection is the CORRECT setting up of these. It took me a couple of years to work out what was going on...
I wear glasses... sometimes... I have a very weak prescription, I can see just fine without them and some people would question why o =n earth I wear them at all! Well... it's because, after a few days of not wearing them, I find myself getting tired quickly, squinting a lot and generally wanting to shut my eyes and go to sleep... all the time! Turns out, all down to eye strain! now, I wera my glasses a couple of days a week or on long drives and the effects are drastically reduced, when I find my =self getting unexplainedly tired again, pop them on and within half an hour all is fine again.. At no time without the speks is anything blurred or noticeably out of focus and I could easily pass an opticians test reading to the bottom of the chart (don't know how long that will last though!) Anyhoo... back to the story..
I could never quite get things right with the binos, something just didn't seem right and I would suffer the same symptoms as the OP... frustrated the hell out of me.. then I realised what was going on.. the binos not only magnify the image, they also magnify any error from the eyeball outwards, in my case by a factor of 8! The diopter only corrects 1 thing and that is focus, it cannot correct for an astigmatism (turn) nor can it correct a prism... so, whilst I could get it close, it was never ever going to be perfect. The interesting thing is, if I wear my glasses and then look through the binos...... lo and behold... perfection.. because my glasses correct ALL the issues with my eyes before magnification so my slight astigmatism is dealt with (which, I think the OP will find is the issue, he has a very very slight one that is no issue normally but when magnified......) as is any other problem..
The downside to this is that I hate stalking wearing glasses, it only takes a light rain or sweaty forhead dripping onto the lenses to really ruin a day and even though my prescription is very weak, it is still uncomfortable to instantly swap between glasses and no glasses, also, glassing wearing glasses is horrid! so... I just put up with it now happy in the knowledge that my £2k pair of binos are fine and well and thay=t the problem lies with my ageing body!!