Scott, for how long are the 115VAC wires and the limit switches together? Am I right to expect they are in the same cable?

If this is the case - this is a bad situation. The amount of coupling is very high due to the capacitance. Most people avoid this due to the problems you are experiencing. My solution around this is to use 115Vac relays and have the limit switches operate these. That way we get away from mixing low voltage and higher voltage signals. Using optos can be a problem, as the coupled energy may be enough to operate them so that means you have to provide load resistors with the attendant dissipation problems.

With you circuit - where is the 5V and GND coming from? If it is from the same power rails as the rest of your electronics, then using an opto is a waste of time. The noise is going to get injected into your GND circuit and will upset the micro.

I had to fix a problem many years ago where a multicore cable was used for high current stepper motor drive and the limit switch signals were also in this cable. The amount of coupled energy in the limit switch circuit caused a LOT of trouble.