First of all, welcome to the forums!
Secondly, to save yourself a lot of grief in the future, before posting, always use the search button at the bottom of the screen first to see if your question has been asked before.
The flickering problem is a topic that has been discussed in great detail quite recently - I suffer from it too.
It tends to happen not on all machines, but does seem to be quite common on faster machines.
As such, there doesn't seem to be a single cure which works for all occasions. As there are a number of things to try, you'll have to find the threads and check the various options out.
The first thing to try would be to add as the first line of your program:
CLS 0: Sync
If that doesn't work try this thread:
http://forum.thegamecreators.com/?m=forum_view&t=109906&b=10
TDK_Man