What you need to do is a split-half search. Basically, you need to figure out if it's an electrical issue or a mechanical one. The electric tests are easier to do and will permit you to eliminate the easy stuff first. The Tune-Up Training Manual (available under literature and manuals-https://packardinfo.com/xoops/html/modules/article/view.article.php?50 ) goes through all of the electrical tests and most can be done with your average multimeter-- that way you can clear the cables, the battery, the starter, and the ignition switch as possibly causing your issues. High resistance in any of these would cause the issue you describe.

I have a 25th series Patrician 400 that has been sitting for the past 8 weeks or so (I've been making some repairs under the hood), and having never leaked before, all of a sudden I started noticing transmission fluid on the ground. First a little bit, and then, as the weeks wore on, more and more. I got under the car this evening to pull the dipstick and found fluid dripping out of the dipstick hole the moment I loosened it -- indeed, the fluid was up to the top of the fill hole and had been leaking out (slowly) from the dipstick. It turns out that when the ultramatics sit for several weeks, particularly if the have some miles on them (mine has 40k on it), the torque converter starts to drain back into the pan over time. If you keep the transmission topped up to the full level on the dipstick, having the converter drain back into the pan will result in overflowing. The solution is to either run the car regularly (even just starting the car and running it in park will cause the pump to re-fill the converter), or to let the fluid leak out and then start the engine, check the level, and refill to the correct level once you're ready to start driving your car again.