I've lived through most, if not all of the problems you've described with your crossbow, on my TAC15i. In my case it was a slow progression of changes that finally got it to the point where it was shooting at six inch groups at 100 yards as consistently as the wind will permit. This is from a bench rested position.
I've proven to myself and other shooters, like Super 91, just how important the matched arrows are, so it's one important step to insuring much tighter groups.
The information u were given by This person from PSE is very wrong and both Super 91 and I have conclusively proven it with our own Carbon Pine Testers and Easton Electronic Gram Scales. The PSE TAC15
Arrows are not a good quality arrow. Their straightness factor is .003 as compared to the .001 of other producers. They have very inconsistent variations in the shaft walls is very inconsistent and worst of all, the shafts are so far out of being perfectly round that if you pull your nocks out, you can easily see how many of the nock shafts have been warped out of round to match the arrows lack of roundness. Last, the synthetic materials that PSE has always used on their nocks is to soft. This allows the nocks forks to spread as the arrows are shot numerous times. You end up with a nock that fits to loosely to your crossbows string.
This is why I recommended changing to the Firenock D nocks and using the Bulldog Collars. The collars maintain perfect roundness of the shaft and provide a great deal of strength to the back end. The Firenock D nocks are uniform and are made on a modern nock mold, of much better materials.
I'll supply more information later on what other changes should improve your groups.