Another option could be your pyrometer being miscalibrated. You're firing to 998o but if the pyrometer has aged or something has affected the voltage across it, then that could be inaccurate. Worth checking on too.
It could also be a clay body difference. Contact the manufacturer and see if anything has been reformulated recently.
I don't believe anyone has addressed the issue that immediately springs to my mind.....
How are you loading these kilns, with regards to shelf placement?
You said that the bottom of the kiln is a full 1.5-2 cones lower... This has absolutely been my experience if you place shelves close together at the bottom of the kiln.
This may not be the case for you, but I just hadn't seen anyone propose that as a cause yet.
Since (on an elementary level), heat rises, the bottom of the kiln will tend to be cooler. You can mitigate this by creating larger "chambers" at the bottom of the kiln throughout which heat can circulate.
But if you have a few 4" layers of plates/flat ornaments at the bottom, with tall objects on top, the top of the kiln will absolutely fire 2 cones hotter.
I started to throw a pot and it collapsed on me, I really like the way it looks this way and i have plans for it, the bottom still really thick, so I need to trim some out, my question is How do I trim this, I dont have a Chuck big enough for it, any ideas? Thanks, and may the clay Gods be with you.
I actually trim things upright pretty often... Especially when I don't have the right-sized chuck and don't want to use a ton of clay to make one.
You need a non-absorbant bat, and just a little bit of water on the bat (I usually just wipe it with a sopping sponge).
Level the bottom of the pot as much as you can, and then (with both hands) rub it in small circles on the bat+water. A lot of people use this technique to smooth out / level the bottom of pots.
If you keep at it, there will be a point where the clay absorbs most of the water and adheres to the bat. It will become VERY hard to move, but you can still do small adjustments to center it.
I trim it upright like this, and then do any finishing on the bottom by hand... Carving out any extra clay on the bottom by hand with foam and a banding wheel.