If it stalls when you hit the brake, then raise the idle until you don't lose any RPM when brakes are applied.

It sounds like your clutch spring has lost some of it's temper due to heat. Pull the clutch bell off and see if the spring that holds the shoes is nice and tight. You can 'tune' the clutch engagement by trimming off coils of the spring and making the engine rev higher before allowing the shoes to engage the bell.

While you have the bell off, take a look at the bearings, and verify that they spin freely. If bearing service is needed, be careful not to leave any excess oil on the bearing that can be thrown onto the clutch bell and cause it to slip.