X2 exactly where i would start...Hey Dano I'm guessing you have already checked the trans fluid level & it's ok. The next easiest thing to check is the vacuum line & hose to your modulator valve. The mod valve is located on the passenger side rear corner of the TH400. Make sure & check this line all the way to the engine to make sure there are no vacuum leaks or breaks in this line. Pull the hose section loose from the valve. If you find trans fluid in this hose then the diaphram in the mod valve is leaking. Even if there is no trans fluid, it could still be bad. At this point I would try a new modulator valve.
If you aren't sure how long it's been since this trans was serviced & you want it to last, you should also pull the trans pan & check for general maintanence. See if the filter is clogged and/or if there is a lot of crud in the trans pan.