Put an VDR in the input ac of your source.
The value of VDR must be a few more than your max peak value.
For example for 220V +-10% means 242V * sqr(2)=342.24V Then tehe VDR must be of 350V.
Change your source for one better. For example an LM323 filters better than 7805.

Put a filter in the line of neon like other VDR or an RC or LC.