I've noticed something in the latest SVN version related to Tech Diffusion I think, if you egt too far ahead in Techs, the cost increases? I went on Marathon Speed from researching Techs at an average of 1 ever 6 turns, as soon as I hit Engineering it went to 38 turns?
I think that it depends on who else has the technology already. If several (or at least one?) other civilisation (perhaps only those with whom you've contact?) already has a technology which you're researching, then it'll be quicker for you to research. If you select different technologies on the science advisor screen, you'll see your effective beaker production amount change drastically, depending on how many other civ's have it already. Perhaps no-one else had engineering when you started researching it, while the previous tech's you were researching were already known by someone.
The way tech' diffusion is currently implemented is, in my view, rather confusing. As I wrote elsewhere:
I think I'd prefer that tech' diffusion were implemented similarly to the tech' learning from captured cities: that is, if you're a long way behind, you get some free beakers added each turn to tech's which are known by other civ's with whom you have contact. In this way you'd be able to judge the trades accurately, and you wouldn't have to open the science advisor to see how long a tech' will take to research. This also seems more intuitive to me: my civ' would start learning about the technology through contact with other civ's even if I didn't apply my own scientists to study it.
Cheers!