Take intractable problem. Abandon intractable problem. Run errands. Return home. Play with coloured pens for a Pomodoro. Transfer clean copy to iPad while Mac updates itself. Accomplishment.
Two key parts: doing something else to give your brain space to mull on the problem, instead of trying to solve it by head-butting a brick wall into submission. And structure your time working on the problem, breaking it into chunks that feel approachable.