I would start with the Engine Performance Diagnostics.
So, the best way is to check the codes in the computer to see if you have a sensor that is going out.
here is link to some good repair info:
http://bit.ly/mitchell_repair_manuals_online
RepairPal Certified shops can diagnose and repair your 1999 Dodge Ram 3500 Van. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.