Not having had a 3/600 generation reader, I can't speak from experience, but if they had the same eink technology screen as the 3/650 and T1, this shouldn't happen, and you may have a hardware problem.....
However, I have a vague memory of someone in Waterstones , when I first started thinking about getting a reader, a salesperson saying, about the 3/650, that they "now have a screen enabling you to read easily in sunlight."
Which perhaps indicates they had improved technology? Or they may simply have been comparing to other readers which didn't have it at that time.
Have you had it long ?
Anyway, someone out there must have first-hand experience - if you have no luck, PM me.
I have had the reader for about 2 years and been aware of this problem for some time. It did however become a noticable problem on holiday on Gran Canaria last week. I have sent a queey off to Sony technical but am still waiting on a reply. Like you I think I have a terminal hardware problem which may not be cost effective to remedy.
I've left it a bit late for a refund unfortunately. I'll see if I get a reply from sony tech, and then probably try the T1. I have considered a Kindle but don't like the idea of Amazon taking over the world.
I wouldn't hold your breath. I discovered this problem almost as soon as I bought my PRS600 because I use my eReader on the boat and so reading in 'direct sunlight' was why I bought the Sony in the first place. I spoke to somebody in Sony about it, and was informed that direct sunlight does not actually mean direct sunlight and that text disappearing is perfectly normal. What a load of tosh, but of course no joy on any refund or replacement as my eReader was a present, bought at my request.
Well that is good to know - I might just invest in upgrading to a T2 because it sounds like it might be worth it. Ufortunately it does not look like it is available in the UK yet so I shall have to be patient - not my best suit