I never saw a mention of a mouse pad, unless I missed it. Is it old, new? Some types work better with certain mice.
Fans really should have no effect, unless you have an absolute garbage power supply or motherboard or there is some extenuating circumstance. Same for the case, unless it's terrible at blocking stray signals, it shouldn't be an issue (have you tried moving it to the other side of your desk?). Bright lights could have an effect if you are getting reflections under the mouse, however odds of that are slim unless you use a white/glossy mouse pad, glass or mirror. Surface of the pad also can make a mouse feel laggy even if it isn't. If it has any drag, I always feel like the mouse is sluggish. I've thrown a few out because they felt like molasses, tracked great, but felt slow. Try some other surfaces if you haven't.
Is the mouse plugged in direct, or is it run through a hub? Which port on the PC is it in? I always try and go straight to the computer, and use the usb port closest to the ps2 port. Those usually have a higher IRQ, or at least used to. I only use those for keyboard and mouse, nothing else. If you use an extension, make sure it's a good, shielded one. Not a microscopically thin Chinese one from the dollar bin. The same applies to the power cord, make sure it's big, thick and shielded, as is a power strip if you use one.
Another possibility is the houses current. If it's a tad low or fluctuating, that could cause some weird things to happen. If you seem to go through electronics quicker than normal, that could be an indication. A filtering UPS could help in this regard, though they actually help in almost every instance in terms of reliability.
Take a look at other peripherals and what they are plugged into, as well as wiring. Speakers in particular can carry a fair amount of power. Are your wires cleanly routed or all twisted into a rats nest? You could be getting cross talk between not just peripherals but power cables as well.
I said at the beginning that case fan shouldn't be an issue, however, is there any older peripherals inside the box, that old DVD rom, sata cable, or case fan, could have gone bad years ago and giving some odd feedback. They shouldn't, but stranger things have happened. Be especially aware if you have any cold cathode lighting inside the box. These are almost always cheaply made and operate at very high voltage. If you have some, unplug them.
Lastly, move the router and modem.