When did you mention these magic 110V solenoids? Obviously this is a critical issue.

I doubt if the voltage spike is getting through the switchmode power supply - unless it has major problems! How are these solenoids driven? Relay? triac?? How does the PIC board differ from your (apart from the cpu). Try to compare apples with apples.

I think you need to give us more information on how exactly your system is constructed. I regularly switch hydraulic solenoids, contactors for 3 phase motors etc with no problems, as do many other companies.

Are there varistors across the solenoids?