When it does it, pull over, put in reverse and power brake it and see if still feels like it is slipping. What I'm trying to determine is if you have a 2-3 overlap issue or direct clutch issue, and the kickdown could not be set right. Did you confirm if throttle is to full position, the kickdown lever is 100% stroked?
Also, verify the fluid level is not low at all. If it's only 2-3, then I'm not leaning towards a converter, but most likely the direct clutch pack is on its way out, possibly. Need the info first, above.