ok make a list of all consumables/standards such as matrix modifiers, acids, graphite tubes, graphite contacts, you use and mark the ones that are new. Replace what you havent yet and verify if it makes a difference.
How is the current lifetime of your lamps or lamp? The instrument must have an option to see these data. If you have an spare lamp available, try it and see if it makes a difference.
Some instruments have the options for aligning the furnace, check manual if yours has this option and see if your instrument needs it.
Contamination in the furnace or in the lenses could be another possible cause. Your manual should have a page for troubleshooting with different possible options, check for this if you havent yet.
Mine has these locks underneath the furnace so before I adjust anything, I open the window for Lamp current and then I see how the current increases or decreases according to the adjustment is being applied. Again, im not sure if all instruments are the same but mine has two locks, one is for front-back movement, and other for angle. it also has a camera where I can see the direction the adjustment is going. Current will increase to highest point possible before decreasing as I keep adjusting
, then I try to find the middle point by finding the highest lowest point of these adjustments. I also mark the position as is now so in case I want to start over.
Sorry my friend but that is the best solution I can think at the moment. Good luck.