My 68 convertible has the th400 in it. Added fluid when I got it and it seemed real finicky, too much fluid and it would go through the different gears without really engaging. Too little and you could feel it engage but kind of felt like it was slipping. Rpms would go up and finally it would start moving. Noticed it was leaking and replaced the tank seal and vacuum modulator. Seems like I fixed the leak but it still doesn't engage very well. Doesn't make any odd noises, just will build rpms when you try to take off then will catch and go but not with any real conviction. Where would I even start to solve this conundrumI?