Sunfire 2.2L

• Automatic transmission
• 49,000 miles

My 99 Sunfire starts PERFECTLY in below zero temperatures, but doesn't want to start in 70 degree & up heat. When I first got it 5 years ago, I thought it was the starter. The dealership I got it from replaced it for free since I just got it like a week or 2 prior, but that didn't fix it and it still continues to stall in the summer. If I start it with the key, I have to hold the key down for about 5 seconds before it starts. If I try to start it with my remote start, it tries 3 times, then gives up. It only stalls after it has been hot and/or sunny for an hour or 2 or longer. I know it can't be because of the mileage (only 37,000 when I purchased it.. only 49,000 today) or being driven hard (I got this right after my first Sunfire was totaled, so I was EXTREMELY careful driving it from day 1). My check engine light isn't on, and there were no error codes when hooked up to a diagnostic tester. I don't know what the problem is, but I wish I could fix it!