WARNING: This page is about a different car, the 2006 GMC Yukon XL, 2006 Chevrolet Suburban, and 2006 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.
Converter Ballooning
Checks
Cause
Converter Limit Valve (214)
Stuck open due to sediment or undersized bore
At High Speeds: Converter Limit Valve Feedback Orificed Cup Plug