It would be helpful to get some more details like is this a single 200A disconnect or a larger panel with more than one disconnect in it, is the service 1-phase or 3-phase etc. First, I'd want to know exactly how hot is hot. I use a Fluke IR for this. If it's over 60 C (or whatever the max temp is for the breaker), I'd take amp readings. Is there an overload situation? Depending on what kind of panel it is, I'd check the connections visually and maybe probe the feeders just enough to see if there's any looseness in them (be careful doing this, if they're very loose they could pop out and cause serious damage). If it's one of those 1-phase GE 200A 4 pole mains on stabs, I would look for discoloration (unlikely if it's a commercial service, but who knows...) and if any is there I'd order a new main with repair kit. If nothing is loose, I'd check FOP across the main (feeder side to load side).
Those steps should tell you if there's an issue with the MB. To check the neutral, you'll want to look at voltage to ground/neutral on each of the legs/phases and monitor for a while to see what happens as single phase loads change. If you see a drastic change (more than a few volts) between the voltages when a load kicks in, it's likely a bad neutral.
