I also have a HovaBator 1588, it's actually running right now. I bought it gently used, and hated it at first. I could NOT get it to come up to temp in less than 16 hours, and it didn't rebound well at all either. I used it that first time out of desperation, and while some of the chicks hatched, I was not happy at all.
When I was cleaning it after the hatch, I noticed there was enough fluff and scurf in the lid that it was worth my while to walk it upstairs and use the vacuum hose on it. There is a small hole with seemingly a small probe sticking out of it and it was really full of fuzz, so I vacuumed over that especially, and wouldn't you know? The next time I plugged that bad boy in (out of curiosity, I was going to mess with it until I was happy or sell it if not) it came right up to temp and has been performing nearly as well ever since.
I simply wouldn't choose to buy the 1588 over the 1602n because I patently do not trust digital thermometers and displays, especially when they're running for 3 weeks at a time in such a humid environment; I'd prefer to stick to analog. But I got the thing because I needed a unit NOW and it was the sort of deal where I needed to get on someone's good side, and they offered it, so it was in my best interest to buy it. So now it functions basically as an overflow unit, and I plan to use it as a hatcher unless something goes iffy with it.