For the past month temps have been above 90 degrees every day. During those summer months my car always runs like crap. Between 2500 and 4000 rpms it will hesitate and studder. The last week at has been a lot cooler, temps have been in the mid to high 70's. My car has ran good all week. Is this just because of the heat or is there something I can do about this.

 

Start checking out your sensor inputs to your ECU with a meter, especially the thermisters (water thermo, AFM air intake, TB intake) and compare to FSM specs...

And of course, hotter air will always create less power than cooler air, all else being equal...