Hi mate,
Its true it could be the remote battery but just try the other spare remote, if this doesnt work as well chances are its more a problem with the system then the remotes, as chance of both remotes not work as same time because of the battery is slim.
I am more inclined to think that its more to do with the system earth. If after driving and hitting bumps etc the earth may move depending on where is was mount, causing the system not to respond. If this is the case though the LED on the dash would not be flashing.
If the dash LED is still flashing and your remotes work fine ie their lights come on when pushed, chances are its some sort of interferance or it may be that the units antenna is not working consistantly. If this is so, then yes it maybe effect by the heat in the car.
If you keep having problems give me a call I own Platinum Car Security, I can have a look at it for you if your in the Sydney area. Otherwise may be able to advise you over the phone
Kind Regards,
Daniel Gardener