How to suspend the flowgragh without any signal transmission for a few sec

Dear all,

how to pause the system for 5 milliseconds? I’m trying to transmit
signal then suspend the system without any signal transmission for about
5 milliseconds. The following is the part of code that I use to
implement this, but it just continuously transmitting signal, and I
cannot see any no signal transmission part on oscilloscope.

payload_13bit = ‘\x01\x01\x01\x01\x01\x0’
while(start_flag == 1)
msg_13bit = gr.message_from_string(payload_13bit)

Could you please help to see what the problem here? Why it cannot stop
the transmission for 5 milliseconds.

Really appreciate your help.