I had a similar problem with my ACER ( Aspire, 5570z, 2008), the first time I installed Ubuntu, but that was three years ago. I think at the time it was a driver problem. I forget exactly what the issue was, but I remember needing to specify the correct driver in one of the X-Org configuration files. I also recall needing to use dpkg to do this. I'm pretty sure that the correct driver was either already installed (just not being used!), or I was able to get it through an Ubuntu repository.
My video card might be different than yours though (Intel 945GM/GMS, 943/940GML Express Integrated Graphics Controller).
Maybe posting the output of lspci would help some smarter people than me figure out whats wrong?
try
sorry for the nebulous answer