I've a TX200HC in .177 cal which has been tuned and given the 22mm skirtless piston treatment, running (I'm told) around 84mm stroke.

The rifle is lovely but slightly low on power and very inconsistent. Last chrono string (30 shots) started with a 10.9/763fps then an 11/768fps but afterwards dropped- 10.7ft/lb down to 10.2, then into the 9's with a low of 9.5 then back up to around 9.9ft/lb.
High fps was 768 and low was 714. I'd expect better than that from an el cheapo Gamo or Hatsan!

Now, the tuner had told the previous owner it would need running in & then trying with some recommended pellets (JSB) and I've now done this.

My question, before I approach the tuner for his advice, is are there any basics I need to check that might solve it? I'm thinking possibly breech seals?

I'm not too worried about power levels. I'd like a little more, nearer 11ft/lb but it's only used for plinking targets so I'm more interested in consistency. Years ago I had a tuned HW80K running on FAC at near 20ft/lb which was ultra consistent with a spread of just 3 or 4fps over a 30 shot string. Really I'd expect similar from this TX.

Any advice welcome, and yes I know 'contact the tuner' is best bet but I just want to cover any basics before going down the 'your work isn't great' route...

Cheers