Help needed for partial digestion of genomic DNA

[Kah-Tong, Seow]

I need advice and tricks on how to perform a partial digestion 
( using Sau3A) of genomic DNA so that I could get a tight fraction 
of 2 to 3 kb fragments DNA. I have tried using a serial dilution of 
enzyme concentration. The result I get is a smear from 0.5 to 4.5 kb. 
Do I need to resort to fagmentation of DNA using a sonicator? 
I have heard that DNA produced by the sonicator method is often 
difficult to clone into a vector.