inconsistant shooting

Just bought a Talon SS. I’m finding that it’ll shoot erratically high and low. Some shots go with a POP ! and others with a PFFFTTT !! with the pellet striking 2 inches lower. The gun prefers Crossman premiers , 14.3 grain over JSP 18 grain pellets. Occasionally I’ll get a good ragged holed 5 shot group but over all, I’m disappointed with the stock gun performance. Being a new gun, is there a break in period where things should improve ? Could there be a problem with the valve not putting out consistent pressures ? It there a “standard aftermarket set up” to get the best out of these guns ? Just to see if it was me, I shot 14 shots into 3/8 of an inch at 21 yards from a bench with my HW100. A common occurance with this particular gun. Pretty sure it wasn’t my shooting. The scope is a Leupold mk 4 with a 50 ft focus adapter. I use this same scope on my HW 100, with Leupold QD mounts. Any suggestions or comments would be appreciated.

Jamie