I know quite a few people have this problem and i think i found a solution.When building your park try not to stay in "build mode" for longer than 5 minutes straight.pause(save if you wish) or go to "skate mode" every few minutes for about 10 seconds and it should help.
I've been using this method and its been keeping me from freezing(I used it last night for about 2 hours and it froze the first time i went about 6 minutes in "build mode" straight).

Let me know if this solution works for you or if anyones found a better one. thanks for reading