Land Rover Repair: Trouble 96 Land Rover Discovery, mass air meter, land rover discovery

Question
When engine gets hot approx. 7 miles on the highway starts bucking around 70mpg. When cold runs great over 70. Have changed all 4-02 sensors, plugs, wires, fuel filter, camshaft sensor, all oem parts and a used vss sensor. Took to muffler shop and they said both cats check good. They also checked for codes and came up with what I do with my checker P0340 Bank 1 malfunction. Could use your advice. Thanks

Answer
This is a situation where fault codes alone don't tell you enough.  If your truck were in the shop at Robison Service what we would do is drive it with the T4 system connected and look at the live data screen.  T4 has a mode where it will highlight any out of spec sensor readings and a drive with that tool would surely reveal what's going on

Without proper testing, you're just guessing and throwing parts at it.  As you've probably discovered, that gets expensive

Could be a mass air meter, a failing coolant sensor, an ignition coil breakdown, a crank sensor or reluctor problem . . . the list is long.