What are the possible causes?
Have you taken any action against false firing due to dVD/dt?
If not, insert the following resistance and capacitance, as shown in the figure below, to prevent false firing.
Between anode (A) and cathode (K) R: 100 Ω, C: 0.022 to 0.1 uF
Decide the values of C and R, taking the operating temperature range and noise environment into consideration.