Hi BFG,
I think we're having the same problem with our 95 Previa (265,000km), and so hope that maybe we can help each other figure out what's wrong. Although I should confess I'm not too mechanical ;)
In my case I am getting a CEL with a code of P0171 (System Too Lean (Bank 1)). I replaced my front oxygen sensor last October, and then at my mechanics suggestion did it again a couple weeks ago, but that didn't help solve the problem. I replaced the fuel filter a couple months ago but that didn't help either. I have an OBDII scan tool and so read live data as I drive. My long term fuel trim value is always at least +7% and can go as high as 24%, which I think is too high, but would love to receive confirmation of that. I know that I need to consider the sum of short term and long term fuel trims, but in my case, it's not unusual to have the sum of these two values above 25% which is too high for Previas, according to the research I've done.
Anyway, this hesitation problem last Fall and it has gotten worse. For us, it's most noticeable on hill climbs when the transmission upshifts into the next higher gear.
From reading http://www.edmunds.com/toyota/previa...liability.html, it sounds like the Throttle Position Sensor (TPS) and TPS Wiring Harness are common issues and so was going to start checking this out when I came across your thread. I guess you don't think these are at fault on your Previa?
Oh yeah, I almost forgot to mention that we also replaced our fuel filler neck in December 2011 and the part wasn't quite the right shape and so our mechanic bent it a bit and made it fit in. And since we were having problems before this, our hesitation issue is not related to this.
Cheers!