I don't really run my car on carpet, but I would suggest that you get the carpet rear tires offered by HPI, and also put a rear spring back on. All cars run on carpet need stiffer set ups. Sometimes, when your settings are too soft, there could be problems. I think that the soft tires are gripping so much that the flex in the chassis is getting to be so great that it upsets the tires grip.
Just my thoughts, but do try a rear spring because I have found that the lack of a rear spring in my micro makes it unpredictable. Also you might want to slightly lossen your rear diff which will increase the rear's mechanical grip. i hope this may help.
I love my micro.