How do you create registrations so they have a smooth transition from one to another ie volume levels ?

I don't generally have this problem on my Tyros-5 (or my previous mid-line PSR's.) I'm guessing that when you created the registrations, they were based on different styles and most importantly, different EFFECTS settings. Reverb, DSP, etc.

I admit I don't have a great understanding of those things. But they can have a pronounced "effect" on the overall sound. And if they're not the same, you will get glitches, dropoffs, sounds that cut off, etc. So if I wanted to fix a problem like this, that would be where I would start.

You might even have to recreate your registration entry by going from one style or OTS to another with "freeze" enabled, to preserve the effects settings. As I said, I'm not an expert but my $.02.