S5 Pro missing images

I know it is a long time since I posted on here. I will chat again at some other time but, right now, I am struggling with a problem and I wondered if anyone has ever encountered this before. I expected Google to show me hundreds of examples of it but I have found none, so here goes.

I am using my S5 Pro in the studio, shooting jpeg. Usually, everything is working properly. I shoot about one image every 3 secs to 10 secs.

But I have now had a strange fault. The first time, I thought it was a fluke or me having a mental aberration but it has now happened twice. Here are the symptoms.

I format the CF card in the camera at the start of the day's shooting.

I am shooting happily, the shutter is firing, and the studio flash is going off, giving me the expected "click, flash, bleep" sequence from camera and studio flash.

I review the images periodically and copy them to the PC throughout the day. Everything is OK. Perhaps I get 700 images, from, say #5001 to #5700, using a 4GB card.

I continue shooting.

I then come to review the images in the camera and also copy them to a PC with my card reader.

I find that the camera has failed to record a substantial quantity of images to the card. So, for example, I might see image number #5837 followed by #5838 but I know that I shot other images between the ones recorded with those consecutive numbers. They simply never got recorded onto the card.

This is corroborated by the person I shot, who was also aware of the images we shot and discussed but which were not visible on the card.

At no time did the camera "freeze". It never prevented me from operating the shutter and firing the flash. It did not report any read errors. There are no blank or black images.

The cards successfully copy to a PC. The PC does not show more or fewer images than when viewing in the camera. But there are still images missing between between shots which have been allocated consecutive numbers.

This has happened with two different CF cards. I have not been able to reproduce the error at will on either card. So I can blast away 100 clicks and get 100 images recorded.

There is no doubt at all that the fault occurred. But I do not know exactly how or why.

If a card were faulty, I would expect the camera to display an error; or the PC to fail to read the card; or for there to be a gap in the image numbering sequence. But none of those things has happened.

If the camera buffer filled up, I would expect the camera to freeze and not fire the shutter.

But the camera shows no errors, it does not freeze, the PC reads the cards and there are no gaps in the image numbers.

I have to do another studio shoot in two days' time. I was intending to use the S5 but I will be prepared to use an S3. I would rather diagnose and cure the S5 problem.

Gas anyone had this same problem before and solved it?