Same phone here, and same problems with sluggish performance. I did a factory data reset and my phone is like new again. It still keeps the latest OS, your app data can be backed up to your google account (the wipe process prompts you to do this), you'll just need to download your apps again from the market (which can be done in one batch). If your contacts are sync'd to gmail you won't lose anything.

I wish I did it months ago. The partition wipes really didn't help. I think it has everything to do with several OS upgrades on this phone since I got it. A data reset got rid of the gremlins.