Hi,
First and foremost never ever ever ever use Application.ProcessMessages(); in background thread and anywhere in general !, nothing good will come from it, and will add more failure scenarios that no one can imagine or think of.
Now to this whole big thread, it is way more complex than it should be.
My suggestion for your case (the code above) to drop it all and replace it with something like the a thread that have one event (not two), single one will do the job perfectly, or even remove the event altogether and use simple Sleep or SleepEx with very short sleep time (interval) like 50ms (1/20 of second), check triggering condition and perform one of the following
1) Enabled / Disabled nothing need to be done, loop again.
2) Exit request, then exit.
3) Need to call or notify the main thread then use PostMessage (simple like that) and loop again.
4) You want more control over timing and intervals then use Sleep(1).
5)
Access the conditions from the threads itself only using.
From the code i deduce you are using this only for Windows then hide this new small thread in small TWinControl (or an invisible form) or what ever you see fit, and implement the a Message handler for that Message coming form PostMessage, and that is it.
On side note : PostMessage will of course use a window message, and most importantly if for some reason the message queue is full or blocked (the messages are not being process for other defect or bugs) then Synchronize from a thread will not be helpful too and will only make finding the real bug/flaw harder, Synchronize will only waste your time finding the bug in almost every scenario were things went wrong.
Also if PostMessage doesn't work or fail then Synchronize will not help either.