Matt, I went through hell with this EXACT problem last winter. Drove me nuts, freaked me out because I thought the worst. Your solution is as simple as a cap change or as complicated as leaking gaskets. Do these steps in order all at the same time.
Search my old post for more detail
1. Check front facia bottle for puncture. If it is currently full, this isn't your problem. Siphon out to fill line on wheelwell with right size hose pushed into the hose going to bottle. While you are at it, remove screw just below bottle and cut the screw in half, file the top round.
2. Put small hose clamps on connections between coolant bottle and front facia puke bottle. You currently have push on fittings between metal/hose transitions. Reports have been made of air getting sucked in during cooling cycle when fluid is returning from facia bottle, which will eventually cause fluid to push out. I bought a small hose package at Murray's for $2.
3. Replace radiator cap with factory 2001 model. It is 1 pound more than yours (which isn't a problem) but most important is that it is a spring type cap instead of a drop type cap. Hold the cap as if it was installed and you'll see the small washer looking seal dangling with a drop type cap. Don't buy that one. I'm not sure what year range the 2001 spring cap covered but that's the one I bought.
4. Jack up right passenger side and burp car. The jacking up really helps. Turkey baster out coolant tank to 1/2 full, get partner to help you squeeze upper hose, 4 hands are better than 2, and don't be afraid to squeeze it a billion times. Always check the coolant tank to ensure it doesn't empty. Fill full, put new cap on, lower. Facia bottle should be to appropriate line on inside wheelwell in step1.
5. Post results because there are more next steps if this doesn't work but I bet it will.
