Actually, Miss K, if you'll check the dialog in that particular scene, you'll see that Spike NEVER admitted being the Doctor. The only thing that he ever admitted to was "keeping them for a friend," referring, of course, to the eggs. Now, THAT, I can see Spike doing much more readily than being an international demon terrorist, especially one named the Doctor after his, Buffy's, and Dawn's experience with the demon Doc only a few months before.
The whole Suvolte thing was just too convenient and over with too quickly. If the eggs and their contents were so valuable and so important to the demon world, there should have been someone coming to check on the consignment when the product wasn't delivered on time. People, or demons, who deal in such things seldom just sigh philosophically and walk away with a sad smile on their faces. They can't afford to.
Obviously, this was a tool used by the writers to break up Buffy and Spike, but it wasn't a very well thought out one. Spike JUST WOULDN'T DO something like that. He might frighten little girls out of their money in dark alleys, but the Suvolte thing was just too complicated for him to bother with. Unlike Buffy, I don't think he was too stupid to do it, I just think he would've found a much less complicated scheme than luring a demon to the hellmouth to spawn, harvesting its eggs, finding the appropriate demon terrorists who would be interested in them, store the eggs (frozen of not), then calmly (well, not calmly) boff the Slayer upstairs while the payload was downstairs. And for that matter, what's with our Slayer? There's a perfectly comfortable bed downstairs just begging to be missed, and she wants to get her jollies on a nasty uncomfortable old sarcophagus? Come on, now. Our girl likes her creature comforts available, even if she never quite makes it to them.
And how is it that Riley just happens to show up at the exact post coital moment that the eggs start spewing little spider things, NOT little Suvoltes, all over the place, armed with an assault rifle and a belt full of incendiary grenades? And where is Sam, who left Buffy to go find her errant husband, allowing the whole Buffy/Spike thing to get most pleasantly out of hand in the first place?
There was definitely a plot here, quite likely a second rifleman, and most definitely a magic bullet...
No, wait a minute, skip that last part. My point is, it was just too convenient an chain of events to be anything but plotted out for the express purpose of splitting Buffy and Spike up.
Now I know what Spike felt like when he was trying to tell the Scoobies that Glory and Ben were one and the same person.