Yes a 98 phase one has one pre car lambda, it could be this that is causing your problem but also check maf sensor too.
I measured the waveform at idle then checked it on acceleration and at 2500 rpm while driving
It was not just the reaction time, when idling the voltage should cycle from around 0.2 volts to 0.8 volts in around 1 second, mine took 3, that's just an example of a slow lambda sensor.
You may be able to check yourself but you will need a fast multimeter, not a cheap one.
Oscilloscopes are not the average diy tool.
If you do have a good meter set to dc volts , connect live lead to black lambda wire, negative to grey lambda wire or a good earth, you should then see it cycle.
While your there check the lambda heater by setting meter to ohms and connecting probes to either white wire from lambda, you should be seeing a resistance of 6-14 ohms when cool.
You will find lambda connector on the bulkhead behind black plastic cover, held on by a single phillips screw or phase two models , it is mounted on the top of the inlet manifold, I don't know about v6's...