I have been able to stay at 240° for 90 minutes, give or take. You've got to realize that offsets need tending, whether they are the el cheapo variety, or a $ 10,000 Jambo.
If you are using briquettes you should switch to lump, there is less ash build up from lump and ash build up will restrict air flow and cause temps to drop.
I cooked without modification to the pit for 5 years, but last year I replaced the charcoal grate and at the same time raised it up out of the as drawer. I also switched from all charcoal with wood chunks added for smoke flavor, to starting the fire with lump and the using wood splits to maintain the fire. These days I cook everything around 300°, except ribs which I still cook between 240°-250°.