I normally aim for 200C or there about when grilling.
Temp will slowly drop over the course of an hour though. I think I normally end up at about 130C after an hour or so. It's generally enough to do what I want.
To be honest, if you want constant heat permanently then gas is probably the way to go.
However, from what I remember your temp is dropping after 15mins. That's not right.
My OTP is about 25C out, but I've heard of 50C.
You need a good digital thermometer to really tell properly. Something you really should have anyway to bbq properly. A so as not to poison anyone and B so not to cremate your food!
Can I ask why you want such a lot of heat for so long? I can't imagine how much food you are cooking.
As said previously pile the coals up on one side under the access grate. You can then top up. It will hold the heat better too.
This may be upsetting after spending a small fortune on your pride and joy,
but ultimately it's just two big bowls with holes in. It can only work with the nature of fire. I wonder if you're not expecting too much?
Don't forget to plan your food too. To me the drop in temp is quite handy as you can start to do more delicate things or crisp things up without a raging inferno.
Good luck with it though.