It seems that heat really kicks the crap out of my car. The car sits in the sun while I'm at work and when I drive home, it feels like I lose about 80 horsepower, with my car struggling to accelerate from a stop...however, when I take it out at night, it runs like a dream. The only thing I've done to the car is remove the air silencer and put in a K&N drop in filter. Anyone else experience such a drastic decrease in high temperatures? Anything I can do to fix this a bit?

 

Vacuum leak. When the car is cold, the vacuums line is also cold and the whole is closed. But when it heats up the hose heats up and the whole expands causing a leak... Happy hunting...

 

Another thing to check is if your compressor is kicking on without having any HVAC on.
If thats the case you could have a bad A/C cycling Switch.

Its located by the black dryer on the pass side of the rear engine bay.

Now if your compressor is kicking on and you feel that much of a loss you should look into having your A/C system serviced, the compressor could be going bad, the system may be overcharged or undercharged. A fault in the A/C system could cause more loss in power.

Here is a pic from a fox, but you get the idea.

 

I forgot to mention I've been getting really bad gas mileage, like 12 mpg with mostly suburban driving and some highway driving. Would either of those problems also affect the mpg?

 

I had a similar problem, the hose going from brake booster to intake manifold, had a small hole in it. i was getting 15mpg before i realized the leak. Simple fix though. After fixing I am now getting 24mpg out of stock 2001 3.8. I hope your problem is as simple, GOOD LUCK!

 

A vaccum leak could definitely cause bad gas milage, not sure about the compressor leak though. Good luck...