I had this problem a few years back in mine B, even before it was converted. Rainsport 1s on the Speedlines, plenty of tread, couldnt figure it out, thought the clutch was dying or something. Turned out that although i had plenty of tread, the tires had covered so many miles that they had just turned rock solid without wearing away the tread. Two solid lumps of un-yielding rubber. Could be something similar with your tires. Phil.