The problems with the IRIS HUD for their F-15 are well known since it came out.  Even if it's working correctly, it's a killer on frame rates on many machines, mine included.  So someone came up with a hack to use the Corral F-15's HUD with the IRIS aircraft model and things are much better now at least on my modest setup.  Now while this may not specifically address your issue, like you are asking, they do have a support forum for their products which can be reached through their website.  Maybe you can find a solution there.

Ever since they released those 4 fighters last week as freeware, their site has been overwhelmed by the downloads, so they've had some site issues since then.