Your issue therefore is - as I already pointed out - quite certainly a physical one. Please follow the link in my earlier reply and check your setup for common sources of interference.

Since (if I remember correctly) Mira only distinguishes between short and long presses in very few cases and whether or not the IR signal gets cut off therefore would not make any difference to its reactions, I'm afraid it can't be used as an indicator of whether or not you're having a physical issue in your setup or not.

Please check your setup (and the room it's placed in) for heat, reflected, direct or indirect light or sun light. If you have the ability to try in a completely dark environment, please do.

Best regards, 
Felix Schwarz