I highly doubt the random variables are heating up your phone. The main reason why a phone heats up is that C2 can only use a single core of a processor - except for a few things that can be handed off in some cases. The more commands the event sheet has to process every tick, the more the phone will warm up. 15 random number generators is not enough to cause a problem. But something else is building up over time and making your phone do more work than you may think.
So, make sure you don't have unnecessary things running - and the fact that you say the game start to lag after a while tells me that more is going on than you realize. Either objects aren't getting deleted, or some condition is true multiple times that is spawning too many new objects, or something like that. It is probably bugs in your code, or too many effects, or too many or too big graphics.