If the Torque Converter slips and takes too much time to engage, it'll time out and disengage. The ignition cycle fix is typical for this problem. You may want to consider changing or flushing your trans fluid. How does the fluid on the dipstick look? Make sure you check it AFTER driving for at least 10 minutes, to stir up all the particles. After settling overnight, nasty fluid can look good after all the friction material settles to the bottom of the pan.
In addition to nasty fluid (and it doesn't really have to be that bad-looking, if the properties of the fluid itself are degrading over time), it can also be caused by corrosion on the trans connector, or even a slow/sticking TC solonoid.