Browsing the said project, I only see snippets, no complete circuit. I don't feel motivated to search links and appended files for something meaningful.
I e.g. don't see how trigger circuit synchronization is managed. Presuming there's a zero crossing detector, that's a popular place where a real circuit might fail due to interferences generated by the load.
You can easily find out yourself by watching the trigger pulses together with the output waveform. Is the unwanted behaviour already present in the trigger pulses, or is it generated by the power circuit?