Worst comes to worst, you may want to consider running your (presumably Flash) game through other means, like Flashpoint.

This is not a Flash game; this is a mobile game which also has an html5 version for the web. That being said, some remarks and suggestions:

About the site:

• You should avoid coolmathgames, and look for alternatives if you can. The site takes a long time to load any game, if you don't use an adblocker then the ads will severely lag your game, I don't know about the auto-scrolling problem but it happens too I guess
• You can play this game alternatively on "CrazyGames": https://www.crazygames.com/game/swing-monkey The game is already on the top of the screen, and if you want you can play the game on full-screen mode

About resetting the game:

• The fact that the game doesn't let you start from the beginning or don't have a level-select, is a bad game design and anti-speedrun in general; Personally I would go away and search for alternate games (there are several games on speedrun.com with the same mechanics).
• You can clear the cache like you did and reload the site, CrazyGames is loading much faster than coolmathgames. However, doesn't deleting the cache also deleting save files for other games too?
• Alternatively, you can look and find the save file of this specific game in your user data folder, but I have no idea where html5 games save their data.