All it does is keep pressure at around 35 psi engine running and up to
about 45 psi with no vacuum (vac hose removed, high load conditions).

Check your fuel pressure, or have someone do it if you aren't sure how
to do it safely.

Toyota MDT in MO

The fuel pressure regulator could cause problems like that. Especially
if the vacuum line to it is clogged or damaged. But testing that is a
bit of a pain, and the first two things I mentioned are much easier and
more likely.

Cleaning the throttle body and idle bypass stuff is probably a good
ideaeven if it doesn't solve the problem. I really think it could be a
big part of the problem, though.

And hooking a volt meter up to the O2 sensor and checking it isn't too
difficult (depending on how accessible the O2 sensor connections are).
The voltage should oscillate between two levels. The quicker it
changes, the better it is. If it's very slow to change levels and it
pretty much matches the surging from the engine, that's a good bet that
the O2 sensor is failing and causing the surging. If it reads a
constant voltage, it's dead and needs to be replaced, but it's probably
not the cause of the surging problem.

--
Racer X

1984 VW Jetta Diesel GT
1992 Mazda Miata (looks OK considering it's been hit by an 18 wheeler)
1994 Caravan (OK, maybe it's a keeper, but I still want a VW Caddy)
1995 Ford Escort (may it rest in pieces)
1983 VW Rabbit GTI (ITB racer)
1988 Mazda RX-7 (maybe for sale, make offer)
1992 GMC Topkick (portable garage for racecar[s])