Hi,
It is probably the buttons on the key fobs. They are notorious for issues. The 'button' (on the printed circuit board) has a quite long centre part (the bit you actually press)
These very often break off about half way causing an intermittent operation.
I do not believe it is anything to do with outside interference at all - (I only offer my 30 years as a Ham radio op as to why)
When issues start to occur to the fob, the operator ends up pressing the buttons harder and harder - this exacerbates the issue.
The answer is a new fob, or - a repair kit (IF you have very good soldering skills !!!!) - these are available on ebay.
You can open the case and look at the 'buttons' - please send a picture of the buttons. Usually it is easy to see when the button operating 'shaft' has broken.
I just had to have a new ''fob' - (supermarket special) - and am awaiting the 'repair' kit from ebay (china no doubt) - however - I DO have an air reflow soldering station - but, I am sure that with care, the work could be carried out with a good soldering iron, solder 'sucker' and braid.
Open the case and send a pic. ?
Yes, of course - it could be another issue - however - having gone through exactly the same 'trauma' myself --- I would guess this is your issue.
Best of luck !.
PS, if the supermarket special fob is recent - take it back to them !!!. - here in Portugal there is a 2 year warranty on ANYTHING new.