Lets not forget putting on cold meat will drop your temps for awhile so don't go opening up the vents to much, let it settle out but try to catch it on the way up as mentioned.
To much fuel, leaks down low have been mentioned but I have to disagree with to much air from the top not affecting anything. For sure if you take your lid off for long its going to flare up so no mopping, spraying, peeking until you are getting close to what should be the finish line. If you do need to open it up be quick about it and have everything you need at hand and never go anywhere like to the store or work unless you know someone that can handle it is home watching it. Make small adjustments with a good amount of time between them. I have just one 2in ball valve on mine and most of the time its is only open about 1/4 to 1 inch except when firing it up or shutting it down.
Get your intakes sealed good, start with less fuel, do a boil test on your gauges both digital and analog. If the temp starts dropping after being in the cook for a time, grab the top lid handle firmly and give the barrel a good shake settling out some ash in the basket but do not adjust at this time because it should start to raise the temp soon after. Lastly if you do think you have major leaks up top then it should be ok to close down the top exhaust vent a little to compensate for the air leaks. Good luck
Lg Spicewine on trailer
Weber charcoal grill
2 ugly drums, semi retired
Bbq toolbox, formerly Chargrill smoker
My wife has gas, its a Brinkman.