1986 Range Rove 3.5 V8 Efi Help Please

I Got This Range Rove About Six Weeks Ago For Offroading (green Laneing) It Run Sweet As A Nut When I Got It After About 3 Weeks It Started Mplaying Up Sounded Like A Lead Was Breaking Down Like It Was Missing So Iv Put On New Ht Leads Plugs Dissy Cap Rota Arm Engine Flush New Oil Fuel Fillter And It Seemed To Do The Trick Till I Got 10 Mins Up The Road I Cant Thing Of Anything Else So I Thought I Would Ask You Lot If You Have Any Ideas

When I Start It From Cold It Runs Fine 1/2 Mile Up The Road It Start Missing Or Sound Like Its Missing I Can Put My Foot All The Way To The Floor It It Revs Up But Dont Do Anything If I Just Push It Lightly It Dose Drive But 15 To 20 Mph Tops And It Missing All The Way Or Sounds Like It To Me

If Anyone Has Any Ideas I Would Love To Here Them....

Thank You All Very Much

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