I see this and its comon, durring tune ups, you notice one header getting hot or failing to get hot at the same rate as the others, naturally thats an indicator that the cylinders not fireing correctly if its not heating up, but if its getting hot faster youll need to locate the source of the problem, badly adjusted valves, leaking injectors, ignition problems, all could be atr fault.
do you have an IR temp gun to measure the actual temp?

this is the one I use (keep in mind you need to read the 1000F-1400F range)
"how can we check for intake vacuum leaks?"
yeah the way I find vacuum leaks on carb OR MPFI intake manifolds is clean and easy with no mess. get a propane torch,or you can use your oxy-acetolene welding torch if you have one, (DON,T TURN THE OXY tank/or valve ON)

(DONT LIGHT IT) just SLIGHTLY open the valve so its allowing gas to flow at a low voluum,start the engine and let it idle at the lowest speed you can then place the tip of the UNLIT torch at any suspected vacuum leak and listen for the rpms to increase and watch the tach, gas flowing into a vacuum leak will increase engine speed.
look for loose or missing vacuum hoses, cracked or broken power brake connections, emmisions system hoses that are loose, vacuum connections to the trans or ignition, loose connectors missing or loose bolts cracked hoses missing assessory connections etc.