Stumped two auto shops....

[shift01]

Hi all, I'm new here and though I might post the situation I am going through right now. To start with, I recently bought a burban (second one) w/72,000 original miles on it. It is a 1988 V10 and ran great when I bought it. At this point in time it has been in the second auto shop for about two weeks now. The problems I was having are: when idleing it was reving up and then dying down to about 500rpm, then reving again, then dying down. It did this hot or cold. When I tried to drive it it is pulsing off the start and through exceleration while the whole time it feels like the e-brake is on and it is pulling a 5000 pound anchor. I have had a little experiance w/ a 87 V20 but I fixed all of its problems. This is the list of stuff checked, tried, and replaced:
Fuel pump, fuel filter, TBI gasket, injectors, distributor, cap, rotor button, plugs, wires, EGR valve, Throttle Position sensor, temp sensor, idle air intake control, O2 sensor, checked pins 15 and B on ECU connection, fresh gas. The code it keeps throwing is "O2 sensor - lean exhaust". Only thing we have not tried is a new ECU. Anything will help. Thanks.