A change in mouthpiece will have little to do with consistency. You're a beginner. Your embouchure isn't well developed, your consistency of accuracy with slide positions isn't there yet, and your air management is probably not there either. For most players these things take a long time to develop. Now, for the sake of argument, all other things being equal, it is entirely possible that because of your embouchure type, dental make up, etc, that a different sized mouthpiece could be better suited to you, but there is no way to tell without hearing or seeing you play. But let's face it. If you're able to get a nice sound sometimes on what you're using, it is possible to get a nice sound all the time. Something else is not happening consistently. Usually it boils down to one of 3 things.. 1. Breathe Well, 2. Put the slide in the right place, and 3. Buzz the correct pitch. One or more of these is probably not happening.
Do you have a private teacher?