seems like it's your FSR.
do a draw test with your battery using a 20A multimeter. it should pull around 450mA when the car is first armed, after 15 min it should become around 20mA. If it doesn't do these, check whether your a/c fan blowing. If it is blowing >> FSR is bad; if it is not blowing >> either FSR intermittently not working or you have another draw from else where. then you have to pull fuses to dig futher.
to make the story short and not to waste too much of your time (because FSR does fail intermittently all the time). if you know the FSR is old (or if you don't know, take it out to see if it is the old model, which is longer), simply change it for $70 if you get it online. if it doesn't fix the issue, then you have to dig further.. good luck to you.