Re: is everyone's tac this horrible?
First up - Your local pro shop doesn't know what they're talking about and is unqualified to touch your crossbow. We've seen just about ever possible problem a TAC15 can have and have managed to fix just about all of them to perform well.
You current have the eyes and ears of some of the best technical owners that exist on the TAC15/15i crossbow front, so they can certainly help and get you going in the right direction.
It doesn't make good sense to try an figure out why your crossbow is doing one thing and not another because it's all guess work without understanding how it's been set up from the beginning.
Please take my suggestions since they are the only sure way to get the job done correctly and see if you have a good shooting crossbow or something else. You need to go all the way back to the beginning as though you were just receiving it new out of the box. Make believe nobody has touched or done a thing to it and you were going to set it up from scratch.
Go back through the assembly process process and check to insure everything is aligned correctly and loosen and re-tighten everything.
Then, go through a strict leveling and squaring process. (If you search this forum, you'll find articles on how to set up, square and level your crossbow properly.
Then, go back through paper tuning from scratch. Don't worry about sighting anything in until you complete the paper tuning process and get the necessary results.
Once paper tuning has been completed then you can ask for help on moving forward, but it will depend on what accessories you have on your crossbow or how things are working at that time.
As you are performing the steps I've outlined above, you can always ask questions if you're unsure of anything.