HELP!!!What is wrong with my car?

[Slvrcamaro00]

I have no idea what is wrong with my car. I think it may be a weak valve spring, but idk much about engine stuff. Okay, the car has around 76,000 miles and it's 2000 Camaro. It started having multiple random misfires last week. I was accelerating up a hill on a hgihway when the engine light started flashing. I figured the wires were wet b/c it had rain real bad the night before, however it happened again and this time the engine light stayed on. I took it in for service right away. The funny thing is that they couldn't get it to misfire, only when they poured water on the wires. Well, $500 later I have a full tune up and assume that everything will be okay. I take it out for spin after getting it back and I could tell it seemed slugish, ecspecially through the higher rpms. Well, it only took a few hard accelerations to get the light flashing. I pulled over and turned it off. Started it up and everything seemed okay. Well, today it did it on the highway again and I had enough. I took it back and they ran a full diagnostic on the engine. All the air/fuel, compression, etc. was fine. The code said that it had a misfire(i don't think it was multiple) that occurs at 5750rpms and at 77mph. They have no idea what is wrong and they can't recreate the situation. They revved it and stuff, but couldn't get it to misfire. *The misfire happens right at readline. They said they were gonna cal their engineers and get it straightened out. The car has lost power from 4-6krpms. Sometimes it is more noticeble that others. Any input on to what is wrong with my car will help. Thanks