Thanks. Perhaps I should have stated at the top of my post that changing from ASIO to WASAPI has cured the popping noise. I would like to know why.
What soundcard are you using?
There are a couple buffers that can be changed with ASIO drivers. Both are accessible in Tools > Options > Audio > Device Settings. The JRiver buffer is under Buffering and the hardware buffer is under Tools > Open Driver Control Panel. I have found that the lower the latency the hardware has, the higher the JRiver buffer needs to be set to eliminate noise. On my Lynx AES16e card, I have to use 100 ms. With another recent card I used, I had to set it to 500 ms.
Hardware buffers can typically be set to 256 or 512 samples. Some cards use milliseconds instead of sample rate. The Asus cards, for example, let you set the rate from 1-80 ms. I use 20 ms.
JRiver should never, under an circumstance, revert to "minimum hardware size."