I meant Samie in my previous post (he is the Previa guru in the PNW). He has seen this many times and thinks it's the throttle body positioning sensor/butterfly something or other. I'm going to take my van to him after I "hiccup" around in Canada for a week for work.

Unnerving is a good description. It's like the engine is going to drop out for a very brief second. And like you, I believe this has been going on since I bought the van with 150K on it!