What I'd suggest to you, original poster, is that you are not experiencing a potential defamation of character suit or wrongful termination, but you are getting the not too subtle hint from your formerly well satisfied employer that you need to be thinking of work elsewhere. The thing is, if they just tell you to go away, they'd be setting you up to be eligible for unemployment benefits while you look for another job. Therefore, they'd much prefer you to self terminate.
There are two ways they are going about this. One is by making you so totally miserable that you'll be thinking of finding another job. Two is giving you such negative and ugly feedback that you're hopefully going to be offended, and stomp off and voluntarily quit with your "pride" in your work intact. Of course this type of quit means that you have a very very small chance of getting to be approved for unemployment benefits if you quit before you have that next job. In order to quit and be approved for benefits, you would have to show that you have a very good job related reason to quit. Your paycheck bouncing, your employer asking you to do something illegal or dangerous, your employer physically abusing you. Those are considered good reasons.
Deciding that your formerly good performance is now rotten and unsuitable and bad and underpar, insulting comments regarding your intelligence, encouragement to your co workers not to respect you or admire you....those are just totally legal management techniques. They would not be considered a good reason to quit the job for unemployment purposes, much less grounds for a lawsuit.
No where is it a rule or a law that your employer has to give you good feedback, or valid feedback, or true feedback or appreciate and evaluate your performance accurately and fairly. They can say pretty much anything they want to about you, true or not.
My best advice would be, since you are, as you say, not fired yet, keep doing the job to the best of your abilities. Do not give them any easy reasons why they might want to terminate you, such as that you come in late or violate their computer usage policies or punching out your supervisor, something like that.
Make them fire you for your so-called bad performance if they are going to. Do not oblige them by quitting and walking away before you have another job to land on. If you had successfully performed the job before, and can state that you are doing the job to the best of your abilities now, and they fire you for poor performance, you file at once for unemployment benefits while you are looking for another job. You will very likely be approved unless they can show that you are deliberately goofing off or messing up at your job for some malicious reason, which of course, there won't be such evidence.
In the meantime, while all this drama is going on, keep records of what is going on, obtain and save at home copies of your former good reviews and performance evals, do your job quietly and professionally, and be diligently looking for another job, not just sitting around fantasizing about suing these rats and making them sorry they said bad stuff about you. Sometimes jobs just end, the employer has the right to do this to you, even after long and faithful service, and your best response is to move on and do better.