High voltage hardware question

[Snap]

You could try and fix it in the software, Make the pic trigger with a known source and then wait a set period of time (eg 8ms) and see if the source is still there. DC spikes are only about 4 ms long and looking for a solid signal could help.
Snap

[Squibcakes]

Just a thought,

You say the buzzer has a big coil in it. Maybe you need a diode on the pic pin to supress any back emf from the buzzer?

Squib

[spuffock]

I have had some really horrible spike problems. A lot of them have been totally cured by fitting a 100n ceramic capacitor across the PIC's power pins, as close as you can get.
Hope this helps.

[GeoJoe]

Here is what I have done.
Bypassing, bypassing, bypassing. Make sure you have plenty of bypass capacitance on your pic power pins.
In really bad situations I have had to put a small inductor on the +5V line between the regulator and the pic.

Good luck

[mister_e]

Ground plane, ground/power routing, splitting power/digital/analog ground, PSU bypassing, PCB layout... tons of possibilities here.

Overloading PICoutput/PORT is still possible. The whole schematic and actual layout could be helpful.

[SOMRU]

Try adding an inexpensive small relay, or optocoupler, in parallel to the coil, then use the contacts as your signal source. Simple easy and I use this type of isolation in industrial welder controls.