First check the exterior variables. Be sure that the brake arm isn't sticking at times. A touch of well placed spray lube on the back of the arm is sometimes enough to help. That is fairly common when someone has to change direction to rectify things. It won't necessarily just stop the machine with no input from the pedal however. If the pins that push the pads against the disc are binding, it can as it warms create more drag however. Beyond that, you'll need to know that the condition of the drive belt is good and not worn thin or worse and that the linkage from the hydro control to the transaxle is secure. Let us know what you find.