It would've been better if you'd given us this code in the first place, and asked if we would modify it, rather than asking for a new code.
Code:
 alias read_next_help {
var %f = helpfile.txt , %lines = $lines(%f) , %chan = $1
inc %help.lread
if (%help.lread > %lines) { set %help.lread 1 }
if ($read(%f,%help.lread) != $null) { amsg $read(%f,%help.lread) }
}


Please note that your code is not the most efficient, and it sends the lines in sequential order, rather than random order, which was part of the original request.