4.0 p38 lack of power

Just took RR out 1st time sinse i brought it, seems low on power, pulls well to start then seems to lack power mid range, its been stood over 2 months and has been re set by Irishrover and his magical box of tricks. It takes time to get up to 70 mph. im taking it to a tune up man, the cat seems to rattle on start up then go quiet after it gets warm, do you think its getting blocked by the cat collapsing? another chap says it may be the air sensor blown due to several back fires in airbox due to a unset LPG, this is being done after the engine tune-up. I dont want to take it to the garage and then pay for silly jobs that i can easy do. anyone have a clue as to what it may be. Thank you.

Just had another thought, when ever i remove the fuel cap to re fuel it takes a BIG gasp of air and i meen a BIG LONG gasp of air. looks to be the green lip that closes after you take the pump nozzel out the filler hole. If it calnt get fuel it would starve it when its under load, it revs fine when in park. Is it a possiabilty that may cause the symptons im getting or a blocked air vent in the cap?

According to the handbook, the cap is meant to vent on the first half turn, and you shouldn't remove it until it stops hissing. That's what mine does, so I guess yours is normal.

Hi Doug...your engine ECU was put into "Adaptive learn mode" and will need to be driven for around 100 miles to enable it to self tune to your fuelling and other parameters. This does not mean that 50 trips of 2 miles each will sort it out a lot quicker than 2x50 mile trips because the engine needs to be at optimum temperature etc,

Hiya mate,
Your Tune-up man should be able to tell you your probs.
If he has proper equipment then he should be able to pinpoint your fault. Nothing worse than guessing, as it cost's more. lol