My Assumpta can be like that. She has triggers (most of which I've figured out) that send her into a panicked rage. I just avoid the ones I can, and deal with the ones I can't avoid. Most of the time, she is absolute velcro-kitty/love bug, but when she tweaks, it is frightening. She was pretty hard to deal with when we first got her (it was sad, because she SO wanted to be loved and cuddled, but she'd just freak out if something scared her), but she has gotten easier to handle over the last 6 years. One thing I've learned is that there's no such thing as a random or surprise attack, we humans just have a hard time reading and interpreting the signs. Assumpta makes complete sense to herself, I just have a hard time catching up with her sometimes.
Introducing cats to other pets is always hard...most experts recommend complete separation and a gradual introduction process over the course of days, weeks, or months (depending on the animals and their progress). When we tried to introduce a dog last year (very slowly, doing all the "right" things), Assumpta reverted to a lot of her previous behavior (fear, aggression) and got quite sick from the stress of having another animal in her territory (even though the dog spent most of his time in a separate room...we ended up deciding that she would need to be an only pet for the rest of her life...though that is OUR decision, not hers. She likely would eventually "adjust" to another animal in her space, at least for the most part, but it seems to be evident that she wouldn't be the "same" cat that she is alone, and after we worked so hard to get her as far as she's come, we just don't want to see her go back to the withdrawn, afraid, hostile kitty she was when she came to us (not even a little bit)...so our conscious choice has been to commit to her as an only pet for the duration of her life.
If you decide to try another cat, do some research on cat-to-cat introductions, and go ridiculously slowly during the introduction process. It's hard for a cat who's defined his territory over a period of years to suddenly have another cat running around in it. You may look at getting an older cat (kittens can be intensely annoying to older cats), and one who is proven to deal well with other cats (most shelters have at least one or two cats who just seem to be able to live with any other cat without so much as turning a whisker).