Went over all the wiring, cleaned ground points and redid some suspected bad connections. After going over the datalogs and measuring signals with a multimeter, it seems that the wideband lambda controller is at fault and is displaying incorrectly. Unfortunately I don't have time for new components to be shipped to me in time, as I am leaving to study in the US. Car has been put in storage and the project will continue later. The future plan is to replace the O2 controller with a Spartan 2 OEM which is a low cost DIY controller designed to run the Bosch 4.9 lambda sensor, the one I already have.

As seen from the pretty graph above, the o2 sensor is measuring changes with throttle and rpm increases. But the digital display and analog controller output is displaying different signals, even after repeated calibrations.