[billybek]

There could be several issues at play here.
If it is a TX (thermostatic expansion) valve system and the liquid line (the small one exiting the condenser) has a sight glass and you can see bubbles in it all the time, it is probably low on refrigerant.
Trane employed a layered condensing coil on a lot of their roof top units. If you live somewhere that there is a lot of pollen/poplar fluff/seed blowing around, the coil can look clean from the outside and inside, but split it apart and there could be a blanket layer in between blocking air flow. This should cause unit cycling on high pressure and will not keep up with the load.
Possible that you have too high a load (too much air flow or too high of indoor air temp or both) and that would cause a TX valve to go wide open. This would result in bubbles in the liquid line sight glass.
I worked for Trane for many years and started with there roof top equipment and have seen many quirks right at start up with some units. We sold a ton (forgive the pun) of these units so finding a few quirks is not unusual.
A good mechanic should be able to find what is going on pretty quickly.
Don't let them run away from a problem that isn't yours.