In a similar situation as 2 below, I found it better just to disable the timer in the timer event and reenable it when the called process is finished.

>2) One problem I faced early on was if the import process was already running when the timer fired - to fix, I added a "busy" flag to the form, and the timer checks this flag before calling the import routine. The import routine sets the busy flag when it starts and clears it when done.