The chain is stretching as it heats up. You're not maintaining proper tension at operating temperature.
Chain tension must be checked and adjusted frequently, especially on a new chain. Here's the procedure:
- Stop the engine and engage the chain brake.
- Check tension on the underside of the guide bar. The chain should fit snugly against the bar but you should still be able to pull it along by hand.
- If it sags, use the chain tensioner (side or quick, depending on your model) to tighten it. The drive links must not come out of the bar groove.
If that didn't work: The guide bar groove may be worn out. Check the groove depth with a filing gauge. For a 3/8" pitch chain like yours, the minimum depth is 6.0 mm. If it's less, the drive link tangs are scraping the bottom and the chain can't seat properly. You need a new bar.
Heads up: After a long session of cutting, if you tensioned the chain while it was hot, you must slacken it off before storing the saw. As the chain cools and contracts, excessive tension can damage the crankshaft and bearings.