I have to agree with you that he doesn't sound right. I mean yes things can go wrong and cause your such problems (with an attached display) however, that doesn't sound right. Unless there is a problem where the display connects to the motherboard.
When you replaced the display, did you make sure to get the exact same one? Or did you get something else? The same goes for the cable. Did you get the exact original or something else. If it is something else then you may have either gotten one that actually won't work with your device, or it has issues.
If it is a different one (not original make/model or is aftermarket) I would suggest returning it and getting the original. For both the display and the cable. Then see if it works fine.
Now if you did get the correct original parts, try updating your drivers to see if this resolves it and also make sure you connections (inside) are correct. If that doesn't fix it, then I would again see about returning the parts and get them replaced. It could just be bad parts.