I totally agree with the previous posters on all counts.
I pretty much keep Lacey on Mare Magic (actually generic "crushed raspberry leaves" off Amazon
which is the same thing as MM, just cheaper) alll the time.
It really works for her but it might not with your girl, I don't know.
The other thing I might do (depending on how much I was willing to risk, etc, and given that it isn't a pain issue) is just ride it out like I would with a young horse. Just get on and truly deal with the issue.
Of course you know your girl but I've found that sometimes that's just what's needed. Often, before Lacey hurt herself+was retired, I would have to have those types of rides with her. She'd decide to pull a trick and I'd deal with it however I thought best. However, as I became reliable with how I dealt with whatever she did, we went from having pretty continuous "ride out" rides to maybe having one a month in a month where I rode 4 times every week.
Usually, I've found that some/most older horses that are inclined to be like this will be, until they're challenged. I've found that once you really push them through whatever the issue is, especially if they do have a bit of arthritis, they'll often seem to realize that acting up did not feel as great as it did when they were younger and that behaving is the thing that really feels good nowadays.
After a ride like that, I would put a lot of attention into post-ride care (hosing legs/liniment/etc) of course. IMO, if she has the comfort levels to behave like that, I would have to say that I think she has the comfort level necessary for a lil' "Come to Jesus Meeting".