Stuttering issue!! Gearbox/torque convertor or engine miss? Please help

[Livefreeordie]

Hi, just bought an 02 4.4i auto sport. Hi there, It stutters under gentle acceleration and the revs go up and down a little when driving steady. I have also noticed poor mpg. I have no dash warning lights at all? Does this sound like i need a tc? It doesnt have any problems in sport mode for some reason? I had oil on my sparks but since replaced rocker cover seals and new sparks but still the same problems. I have just bought the car in the last few weeks and i am gettin frustrated with this! Also, forgot to mention it is lpg converted/duel fuel but i experience the problems on both gas and petrol. This is making me think its a gearbox issue. I would be very grateful if you could share any opinions. Thanks, Gavin