>

>lcstring=FILETOSTR(filename)
>if OCCURS(CHR(13)+CHR(10),lcstring)>5000
>     =strtofile(substr(lcstring, at(CHR(13)+CHR(10),lcstring,1000)+2,filename)
>endif
>

You beat me to this one... but then, it's not perfect - we are relying on the EOL marker being CRLF, but the file could have been edited with something that leaves only a CR (OK, nitpicking, I know). So just to show that the usual number of ways to do something in Fox is not always three, it can be much more:

lcString=filetorstr(filename)
set memowith to 200 && or as needed
n=memlines(lcString)
if n>5000
   cl1001=mline(lcString,1001)   && the first line to keep
   strtofile("Truncated at "+transform(datetime())+;
      substr(lcString, at(cl1001, lcString)+len(cl1001)), filename)
endif