The overhead camera radar is a fairly substantial drain on performance, could you make a 3D or 2D one instead? - I mean personally I'd rather see an Elite style radar, as it's really clear when things are above or below, something along those lines would save you a lot of cycles I think.
That's what I'm thinking is causing your issues - if you consider that with 2 cameras, you have 2 possible object screen locations, so you might have to set the current camera back to 0: SET CURRENT CAMERA 0.
Then any screen position calculations should be done on the main camera, but if you can get rid of the extra camera, then you'll be much better off I think. If you wanted, you could even check out ImageKit, which allows you to draw directly onto an image in memory. Like you could have a cockpit and the radar screen as an object that you texture with the drawn image.
