Chainsaws

The chain flies off the bar on my new PP4218AVX 18" PoulanPro every 4 or 5 cuts. If i get 6 or 7 cuts, it's a long run. Then, off comes the chain. The saw has the tool-less tightener. That seems to work and does tighten the chain. Have tried everything i can think of on this, including tightening the chain after every cut. Have put on the second chain that came with the unit--same problem. Have only had the saw a month or so and used it twice, so it's new. This is a suburban home so it's light duty. It's really quite frustrating and though i started out thinking highly of the company and the saw, am starting to think less highly.

Any ideas?