BF Turbo doing my banana in.

[SeducedBF]

Ok, here goes. Hopefully this makes sense.
I'm looking for some advice and/or assistance with my BF turbs.
For a while now it's occasionally had an overboost condition that's been fairly manageable and not of any great concern. Sometimes it trips the system into failsafe and I get the code P1227.
I had the solenoid tested early on an it tested o.k. (apparently) but I've always been suspect that it's a bit sketch. I've replaced the wastegate solenoid valve and the flapper mod is done but the old girl still overboosts and I'd like to rectify it.
But wait, there's more.
Recently I was happily motoring along when suddenly the old girl started chugging like a German at Oktoberfest under boost only. Basically, I'd starting boost (irrespective of throttle position) and she'd starting misfiring and jerking all over the shop with some backfiring for good measure. Again, this was only under boost. Drives a treat if I nurse her along with pumping her full of that sweet, sweet boost.
Now obviously that's a problem. 

Plugged in the scan tool and got the P0420 code. Bit odd but ok. Checked the O2 sensor readings and they were.... not great.
Values were not in line with what I pulled out of my Workshop manual so I ordered two replacements and changed them.
Guess what, didn't solve the problem but here's the kicker, when she's cold (not at operating temp) I can hit boost and she pulls like a billionaire at a....... well, anywhere really.

But as soon as she warms up, it all goes pear shaped and she carries on like a pork chop under boost again.

I've tried a few things and have no other reason to suspect my cat is no bueno so I'm a bit stuck as to what to do from here because I don't really want to throw money at a cat for now reason (as if you can even find a standard replacement anyway).

It's worth mentioning that I have a catback 2.5" system on it. Dump pipe and cat were standard ( I have a 4" and 100 cell hi-flow that can go on but was holding off until I can get a tune )

I'm after any thoughts about how to proceed because I'm at wits end.

[k31th]

it does sound like a catalytic converter issue... but it could be related to the thermostat activation, too.

[SeducedBF]

Ok, that's got my attention. Can you elaborate some more?

[k31th]

well you said it only really stuffs up when it's warm/hot... I don't know what might be causing it downstream of the the thermostat but that's somewhere you can get a specialist in these vehicles to check on it.

[SeducedBF]

Not a bad idea, I'll look into it. Thank you!