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I had the same thing happen to my 04. I don't think it is the harness because I believe problems with the harness connection would leave the solenoid in the engaged position, but others may have more information concerning that. I had to have it replaced at the dealer. My reverse syncro was also 1/2 gone. At high RPMS it didn't seem to take much to grind the syncro. I would take it in. I would attach the CAGS harness if you have it disconnected before going back to the dealer.
The dealer can tell if the solenoid is not working properly with their scanner. However, not much question that your's is out. It is fairly easy to change and does not require dropping the tranny, but it does require a special, expensive tool. I would take it in for sure, but if they talk about dropping the tranny I would question that.
This was happening last year and the year before with my 97. I took it to a dealer just before the GMPP ran out and was able to actually demonstrate it happening out on the highway although it had only happened on the track before that. They ordered a new solenoid and when they were putting it in decided to compare the old one and the new one. They were both fine and worked exactly the same. The problem was that I was using too much force when shifting into 5th on the track. The solenoid is designed to be overridden if a lot of force is used. This provision is in case the solenoid fails and the car cannot be gotten into reverse. There is a very stiff spring inside the unit that can be compressed to get the shifter into reverse. They installed the new unit but said I would have to pay more attention to how I was shifting. It may have been the spring in the old unit lost some of its tension when the car got really hot but I never got to try it again since I bought a Z and sold the 97 before I got back to a track where I could get into 5th.
Bill