Apple released updated versions of the iMac in early August, and early customers have now had time to test-drive their machines for some time. Now it turns out that some of these have been affected by a graphics problem.

The common factor in all the reported cases is that they affect the top-end model with AMD’s Radeon Pro 5700 XT graphics card.

(This may make the issue relatively uncommon. The 5700 XT doesn’t come with any of the base configurations – it’s a £500/$500 build-to-order upgrade on top of the i7 model.)

At time of writing, a forum thread on MacRumors has 429 responses; in an associated survey, 26 users answered that they have the problem. Two of these say the problem disappears when they connect an external monitor to the computer.

There is also a thread on Apple’s support forum; this hasn’t got as many responses, but 37 people have clicked ‘I have this question too’.

Most people affected users report a white horizontal line that appears from time to time, seemingly completely at random. But at least one user has encountered a still stranger error: the corners of windows are rendered without transparency (that is, without rounding) in some positions but not in others. A video shows what it looks like when the window is slowly moved on the screen.

Some users report that the error is not visible at all in Windows, which indicates that there is a problem with Apple’s drivers.

We didn’t encounter the issue in testing for our iMac 27in review, but our review sample came with the Radeon Pro 5500 XT. For general advice on sorting out Mac problems, read our guide to fixing a Mac.

This article originally appeared on Macworld Sweden. Translation by David Price.