An option might be to make an array, and everytime you want to add something to the event log you add it to the array (in the listener thread). In your main function there is a timer that checks the array every ... seconds (10 or so) and writes every line to the event log, and then clears the array...
It may be a strange option, but it might work. Although I'm sure there must be better options!
[C#] EventLog and Threads
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