Dreaded CEL - what next

I have a 1997 Rodeo (3.2 SOHC). For the last six months, my mechanics have dreaded to see me coming...it's always the same thing, the check engine light is back on. The latest saga is that they had my car for two days and couldn't find what was causing the Code P0133. In the last six months, the following has been done,

1) Replaced the Bank1 Sensor1 O2 sensor
2) Replaced the same sensor again
3) Replaced the fuel pump (pressure registered below spec)
4) Replaced the exhaust manifold (had vacuum leak)
5) Replaced the Bank1 Sensor1 O2 sensor again
6) Checked the resistance on the wiring to the replaced sensor with an ohm-meter (checked out fine)

The darn light's back on...after driving only 30 miles.

Anybody got any ideas out there? Do I need a new PCM? If I do, what's it going to cost me? I don't want to buy a new car, this one's only worth 2200 in trade and I've spent that much in the last year.

There maybe nothing wrong. You may have just tripped over the 100 000 k mark and then the check engine light comes on to make you go and replace your o2 sensors. Your o2 sensors may have still been giving off good readings. The only way to get rid of the recurring check engine light is to switch a pin in some instrument panel.
There is a write up but I don't know where it is. Maybe someone can post a link.

John