I have a 2001 5.9 Durango and I've been having problems for a little while now. When driving at highway speeds, the engine revs itself 300-500rpm. No real loss in power per se but it is consistent at random intervals, if that makes sense. Just today it started doing the same thing while idling and even came close to stalling. Is this the tranny slipping or a fuel system related issue? What are my options for troubleshooting?