I have a really annoying problem and I'm running out of ideas.  
The engine in my BA XR6 Turbo will suddenly loose all power then regain power and run perfectly. It's really sudden when it happens. It doesn't cough and splutter or miss. It's just all or nothing and will do that a few times before it settles down. 
When it happens it can be for a split second or many seconds so you have to roll to a stop. After a while it's all ok.
There are no error codes reported by the scan tool so the PCM doesn't even know it's happened.
One thing is consistent. If I go for a drive after the car has been sitting for many hours it will behave faultlessly. If I drive say 20 minutes or more so that everything is properly warmed up then let the car sit for about 20 minutes or up to a couple of hours, then it's almost guaranteed to play up. After about 5 minutes of driving with it playing up, it comes good and then it's perfect. 
It's doing my head in. Anyone seen this before?