Many thanks for the responses. The fact that the same problem occurs using DVI on the 2nd PC with a different cable but works ok using an HDMI connection on the same GPU, and both DVI ports on the GPUs work ok with my old monitor, I think it pretty much proves the problem to a faulty DVI port on the new monitor. The fault was probably there from early on but wasn’t noticed because until now, the monitor was only ever used with HDMI. Of course the 3 year warranty expired just a few weeks ago and I'm waiting to hear back from BenQ whether they are still willing to cover it. If not, I will need to upgrade the GPU to one with DP ports that will support 2560x1440 (I want to use the HDMI connection for the Mac Mini as it always has been). I'll probably get the GPU from eBay, I started looking just in case and thought an AMD FirePro V4900 1GB might be a likely candidate...?