Bruce,
There are two things that you have to check.
1: Connectors. inspect your connectors for dirt, debris or damaged pins. This can cause the screen to display improperly.
2. Which display you bought. I don't know your source for displays, but you have to be 100% sure that they are sending you 3G displays. a 3GS display will fit the connector but will not run correctly. Then given that, you unfortunately have to be sure that you are receiving a quality product. Do you have another phone that you can use to test the "defective" screens with? It's a good way to isolate the problem.
Hope this helps, let us know what you find.