Oh, well there are several things that can cause this problem. You are going to have to use trial and error and experiment to eliminate possible causes and narrow it down to the problem.
The most likely causes (in order) are:
1) Problem/error with the model
2) Problem/error with your Hammer settings
3) Problem/error with your graphics card
Please answer the following questions to help solve the problem:
- Has this always happened to the model?
- Does this happen to other models? If not what makes this model different?
I suspect this is a problem with the model. Did you modify the model (are the textures in the correct format)? What exactly happens when this model is viewed in game? Does the model have the correct settings in the QC file?