Improve Reading Of A File

I read a text file every second..[code]The file is generated from another program and it's costantly updated more times per second.The sense of the code (StartIndex) is to read only the rows that I have not read from the last time.The code works without problems until the file is about 2.5 Mb in dimensions... for higher dimensions I have problems and all becomes very slow.Using and fs.Dispose() are like Microsoft suggest (and in fact if i run the code analisys I have no problems), but surely the problem is not here.The question is: is my code right or there is way to improve the reading?? Is there way to read only the tail without to open the whole file?