Q: mid 2009 macbook pro 5.4 problems

i'm trying to keep this thing as long as possible, but i'm getting discouraged.  it's overheating, display is enlarging out of nowhere over and over again, trackpad is not moving the mouse well.  i had some additional ram installed, but that hasn't helped, though it is running a little faster.  i had a look inside, it's not dusty and the fan doesn't look dirty.  trackpad clicks down so i'm assuming the battery isn't swollen, though it probably needs to be replaced at this point.  i'm running a virus scan - OS X malcol was detected this evening - don't know how much of a problem this is.  i upgraded the operating system to el capitan.  i did buy a mouse for it and it seems to be working okay, but i don't know if that did anything to my MBP.  screen enlarges with or without the mouse connected.  display enlarging and shrinking by itself as i am typing this!  crazytown!  any ideas what might be going on here?