You were taking the drug long enough, and in sufficient dosage, to beging to adapt to it. That's all it takes, apparently, to set yourself up for withdrawal symptoms.

At this point, I wouldn't recommend going back on the Effexor. You're well into withdrawal. I've heard many people say that the OTC med Benadryl (diphenhydramime, also available very inexpensively as the generic at places like Walmart) is a good treatment for the brain zaps and such. You need to be aware that the Benadryl can make you quite dopey on its own.

In time, this will fade away. Right now, it's probably scary to experience. If you can try to remain calm during these episodes, at least you will minimize their impact on you.

Best of luck with it.

Lar