The TH700R4 is actually a pretty good transmission. I personally think that a lot of the criticism of it is a bit unfair-- people putting it behind "built" motors making more torque than the tranny was set to handle in stock form, etc. I've been towing with mine for years without problem. Of course, I've also changed fluid and filter roughly every 2 years/25,000 miles, too.
The shifts are a bit crisper (particularly the 1-2 shift) these days, actually since I installed a tranny cooler. But, I haven't had problems and the thing keeps soldiering on with 130,000 miles/18 years now.
Like Mike suggested, check the Throttle Valve (TV) cable. Changing fluid and filter couldn't hurt, either. Other than that, it's really an internal thing, something that's above the head of the occasional weekend wrenchhead like me...
Good luck!