When the Client computer graph shows that nearly the entire bandwidth is utilized for the full test duration of the send session and the receiver shows that after a number of seconds into the test a drop from nearly 100% utilization to 0, the most likely cause is that a router between the PC's is dropping UDP packets.
UDP is not rate adaptive (basically packets are pushed out as quickly the PC can push them out with no consideration whether the packet was received), unlike TCP. A router between the PC's could become congested and hence start dropping the UDP packets, and remain congested until the end of the test (This is of course more likely if the router is a lower end router or there are other users on the network). Depending on your router, you may be able to get UDP statistics on the 'in' and the 'out' ports to confirm this. You could also try connecting the 2 PC's directly (depending on what you are trying to test).