Poulan 380 being used to cut 20 inch ash tree. The bar it came with was 24 inch and carried a 84 link full chisel chain. The saw ran fine for two hours. Then it threw the chain even though it had been adjusted twice after break in. The links were damaged to the point that the chain could not be pulled through the bar by hand. I was 104 miles from the house so purchased a new chain [3/8 .50] from oregon. New chain lasted 5 minutes and it was off. Was told to replace the roller rim sprocket and I have done so. The bar appears straight. Is there anything to check for before starting again? The saw is used with unknown hours.