Gears randomly shifting?

Gears randomly shifting?

Not sure what the issue is here but hoping for some ideas/advice...

I am riding a 8 speed hybrid (3 x 8) bike.

When I am pedalling along (typically in 3:4 or 3:5) sometimes I experience the bike shift from a comfortable high'ish gear to a very low gear (on it’s own!). After almost losing control of the bike:

• Sometimes it returns immediately back to the high gear (on it's own!); or
  
• Sometimes it remains in the low gear for several revolutions before going back to normal; or
  
• Sometimes it remains in the low gear and I need to change up/down to revert to normal.
  
  

I have tried to repeat the effect however I cannot. It seems to happen randomly.
It does not look like the front chain ring is changing but it may well be something happening on the rear cogs or derailleur?

The chain was replaced (and not the chain wheel) a long time ago. There was some chain slipping in 3:4 but this stopped happening a while ago. The problem happening now feels and acts differently.

I did a search around but not sure how to technically describe the issues.