Transmission has a governor of sorts......it is sticking... try flush and fill, w/filter.
also, check vacuum lines under truck, that are ON TOP of Tranny body. they tend to work loose, if in harsh weather.
RepairPal Certified shops can diagnose and repair your 1999 Dodge Ram 1500. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.