I was wondering if any of you fine people would help me solve a few issues my recently purchased E34 540i 6 spd touring with LPG is having. The problem is regarding performance. Generally there is lack of power from the car and accelerating hard with your foot to the floor results in a sudden choke on performace at about 4-5000rpm with slow acceleration there-after. This is both in LPG and petrol.

There is a slight misfire from the engine which results in some rough idling and a rattling coming from the underside of the car on startup which could be to do with the cats. Any suggestions would be appreciated.