Just a good tip of me just buy an good cpu + Ram and your problem should be solved ^^
but like u said that u sometimes won't get stable 60 fps.
I've got an i7-6700 with 24gb ram
and a 550gb ssd and the game is running with avg. 30-40 fps max in high Settins. (2d)
I would Recommend to try a Benchmark to see Which Browser should be good Enough for flash Games.
Here is a link of a benchmarking website where u can try it out.
Link:
btw a pic of some user who already tried every browser each with benchmarks.
Credits to Kalibro8
Link:
edit: try to clean your browser history (cache & cookies)