How To Get Time To Extent Of Exact Millisecond

I am developing an application that is very very time critical. I want to put a log writer in the code so that i can track the events and find out where the application is wasting the time.But the problem is that vb.net tells the time only at a interval of 15.25 milliseconds what to do. although msdn documentation says that we can get the time in millisecond.
To understand my problem better pls go through the following code. this code is written on a click event of a button and it writes down the time in a log file. The problem is that it writes time like 16,16,16 and after some time when the dx value reaches to say 5000 it jumps to 31 and then to 46 and so on.

dx = 1
While dx <= 100000
dx = dx + 1
SRLog1.WriteLine("dx vALUE > " & CStr(dx) & " Ticks -> " + CStr(Now.Millisecond))
End While