Put a new front O2 sensor on the '99, 219K miles.
A couple of reasons for doing this. First: The original nuts holding on the O2 sensor (possibly the one installed at the factory? I've owned it since 155K miles) finally crumbled into dust a week ago, and the O2 sensor popped off of the exhaust pipe when I was about 2 blocks from home. No CEL while it was dangling in open air (really, Toyota? Seems fishy...).
Secondly - I have been noticing a sort of wavering loss of power mostly noticed when the engine is cold and you're doing part throttle acceleration. Not missing or running roughly, you just sort of feel the engine going through weak 'patches' in the RPM range. If you put your foot all the way on the floor it pulls smoothly (I'm assuming the ECU goes open loop at WOT?). Sort of like the mixture wandering around out of spec.
Anyhow, brand new sensor on (with copper nuts) and the wavering weakness is gone, it pulls smoothly when cold at any part throttle, no noticeable improvement for flooring it.
Now to wait to see if the MPG improves on the next tank of gas.