I'm also having extreme random drops in fps. Mine happens regardless of first or third person tho, and it's constantly every few seconds. This is the same kinda crap I had to deal with in update 1.5. Why is it that they seem to ruin the game everytime they patch it? Wasn't one of the things they supposedly fixed was memory usage and stabiility? I had it running perfectly again right before 1.8 rolled out and now it's back to being unplayable again. This is really pissing me off at this point.
My lag happens only in first person mode and one thing I forgot to mention (even when editing) was that it only happens when moving my mouse around in first person mode.
It's been getting better the longer I play it but there is still a lot of lag. Also, I tested interiors, now with interiors (not dungeons, I talked about those in the OP I'm talking about houses and such now) there is a major almost halting the game FPS drop for about 1.5 seconds, after this everything continues as normal with no FPS drop at all no matter what camera view I'm in (first person or third person).
On top of this, the halting drop happens upon entering only sometimes and when it does happen it doesn't matter if I am in first or third person mode. The times it doesn't happen I don't get any FPS drop in first or third person upon entering nor walking/looking around.
Yeah, weird stuff and I am slightly disappointed.. though oh well, it is a few million lines of code so you have to expect some bugs when patching I guess.. their QA team just svcks though, but that has been known since they released Oblivion.