It's a very good time to be an Arsenal fan and just like you said, Arsenal have proven to be unreliable in recent years. Nonetheless, I think there's a different feeling about them this year.
While we may feel it's a two horse race between them and Tottenham, I think it's not over yet for Manchester United (no sarcasm intended). We all saw how we thought Manchester City could run away with the title and now they're just six points ahead of Liverpool having played a game more. It takes just two to three games for the dynamics to change. United even at their worst can still grind out some results; remember they say class is permanent while form is not.
If with five matches left and things are exactly like this or more in favour of Arsenal, then I can totally believe United are out of it.
I totally understand you. I've just been disappointed over this United side for too long. They always miss the chance to move up or close up gaps and it has been the theme over the years. You are right... It only takes one or two convincing wins for the dynamics to change and I hope for their sake, it is true.