I'm definitely with pastrygirl on this one - sounds like overcrystallization. How much seed are you adding to your 45C chocolate? (I assume you're not tabling ;))
Can you describe your tempering technique? If you're seeding, there's absolutely no need to do the 45-27-32 temperature profile, simply add the seed and stir until it reaches working temp (I know a lot of people on here don't do the stirring, but it works for me, and I am quite OCD about things when they work :P)
Most of the time I am using a Chocovison machine for tempering. I followed Peter Greweling's suggestion for adding cocoa butter to thin out chocolate that is too viscous--so I don't think the mere addition of cocoa butter should make the choc. tend toward overcrystallization. The machine calls for putting in the seed choc. after all the choc. has melted; it lowers the temp. to 90 degrees F., then tells you to take out the seed, then continues to lower the temp depending on the type of choc. I would estimate that only a few ounces of seed melt. The machine allows for the possibility of raising the final working temp of the choc., and I plan to try getting dark choc. up to 90 degrees F. (Callebaut recommends higher working temps on the extra bittersweet package). Yesterday I tempered some dark choc. by hand, using the partial melting procedure that Greweling also mentions--removing the choc. from over the hot water before all was melted, then stirring to melt the rest. I did, however, find one glitch in tempering choc. by hand: by the time you test the choc. for tempering, its temp has lowered. In any case, that batch of choc. was like the machine-tempered choc. The viscosity is fine when I am spreading it on the mold; it's just that it coats the scraper rather rapidly and becomes firm so that it's practically impossible to clean off as quickly as one has to operate. I don't know what is causing this problem, though I agree it sounds like overcrystallization. It's certainly not that it is sitting too long in the machine because I make one mold at a time, and as soon as the choc. is ready, I pour it into the mold. As a temporary solution to the utensil mess, I am going to have several scrapers ready. And Pastrygirl's suggestion that the scraper be positioned perpendicular to the mold (although not what most people say) seemed to help. Also helpful was the idea I found somewhere of scraping half of the mold, then reversing it and scraping the other half; in that way you don't drag the choc. across so many cavities.