Diagnostics on P0420 code

[MadMalx]

The engine management light has been coming on for a while now on my 2003 Focus (1.6 Zetec SE engine), and after a bit of searching the internet I have found this could be either the lambda sensors or the cat. I used my diagnostics software on the car and found that once the engine had gone into 'closed loop', the pre-cat lambda was switching but the post-cat was giving a steady reading. Problem is that after a few days, when the light had come on again, I re-did this test and found both the pre and post cat sensors were switching with a very similar pattern, indicating a failed cat. Am I right in assuming that the cat might be borderline failing hence giving me the different results?