Strange behavior of a 565 cal. Please advise.

Hello, I have a Semaster 300 (watchco) with vintage 565 calibre from 1970 on rotation which started to behave strangely… First running 2 mins/day fast, then keeping perfect time, then losing 2 mins/day – all just in few weeks.

Before it was running strong for a few months and seller promised it was checked and no needed service yet.
But a week ago after wearing it I saw it was running about 2 min./day fast. I checked it with Hairspring app with ok result (like +18sec) but physically it was gaining about 2 minutes.
After few days I decided to check it again. Surprisingly it was running accurate, just 2-3sec/day so I started wearing it for a few days again.
After some days resting in a box, I picked it again and it is losing 2 mins/day now.

What can it be? Seems like the first minute it loosing (or gaining when it was running fast) already in 1 hour. Then it runs better.

Am I the only one who thinks a trip to a watchmaker might be in order?

Not without proof.

Thanks so much! Sure I wanted to go to watchmaker right after I saw it run faster, but then when it started to show correct time I was confused. So I was thinking maybe somebody will explain this as magnetism or whatever could it be. Now I remember the day before I saw it started to run fast I was wearing it on some event standing quite close to big sound system for instance...