heat issue on ECB - water brinkman bullet

The minion method could help you a bunch, try it tonight. It could easliy fix your problem.

Minion method is best for that kind of smoker.

Raise the coals higher off the bottom, so there is more room for ash to collect and not smother the fire

Try kicking the bottom when it starts to temp dive to knock loose ashes down away from coals.

Use a high quality charcoal briq such as Stubbs, (avoid Kingsford blue and other el cheapo briqs) or use a decent lump charcoal packed in tightly for better burning.

Drill a bunch of holes in the charcoal pan on the sides for better airflow to the coals.

Make sure you have a way to choke down the air intake for lower and slower cooks. Personally I'd be jazzed having it cruise @300, more sleep time.

I load my ECB (electric Gourmet converted to charcoal) with 1 full chimney worth of briquettes and using the Minion Method it'll hold 250F close to 10 hours. Lid is sealed with rope gasket and the 3/4" vent holes I made on the lid are always fully open.
My charcoal pan is well sealed with one single adjustable vent.

Also make sure you're not getting any air leaks through the access door. A leak there will keep temps high no matter what you do.