Ryobi EBS 1310V Belt Sander Fault?

Hi hope you can help,
My Ryobi belt sander has developed a fault it seems to work ok for a while from cold, but as it warms up from use the motor starts to faulter ie slows down speeds up and runs intermitently the carbon brushes dont seem to have excessive wear about 10-15mm left.
There is no smell of burned out motor and it doesent seem to get excessively hot when running any ideas?. thanks