It would not hurt to check that the spring that the idler uses to tension the belt is not weakened. If the belt does not have enough tension, the belt will slip, get hot, get hotter, and finally start to smoke. It would take some time for the belt to get hot.
I am not sure whether you have replaced the idler pulleys yet, but spinning the old ones will give an indication of their condition. If they spin freely but make a bit of noise, they likely are a bit short on grease. They can be greased by forcing grease behind the seal or shield, or by prying the seal out, adding grease, and replacing the shield.
If you have replaced the belt, be sure you got the correct size as being oversized can lead to slippage, and undersize can add excess load on the idler bearings.
tom