2002 disco, 93k miles.
problem: only getting about 13 mpg, used to get around 15-16. random blinking check engine light, three amigos and misfire during acceleration. losing about a tank of coolant every month or so and misfires from about 55-65 mph (engine just kinda chugs along for a 10-20 seconds) otherwise runs smooth.

NEW over the last year and a half-
heads and head gasket
water pump
main radiator hoses (no obvious leaks anywhere, but still losing coolant)
new spark plug on 4th cylinder and all spark plug cables have been replaced with land rover brand cables
brakes and rotors
tune up & air filter just over a month ago

coolant system pressure has been checked and was deemed to be just fine. Transmission checked and also cleared of any problems.
Same as above with the ABS system.

Can someone give me some suggestions as to what my problem or possible solution may be? this misfire is my biggest worry at the moment.

 

Would help if you had codes read and list them by number, not what scanner thinks they might mean. Like P0174. No need to take into the shop, an auto parts store will usually do this for free. Check engine light may be intermit itself, but scanner will show "MIL Status = ON" for light should be on, and codes will be present. Were the other seven dwarfs - I mean spark plugs - changed at some recent point (did tune up include new plugs)? While they are checking your codes, ask them to note the coolant temperature from the scanner. Where does your temp gauge needle point after you have driven the car for 15 minutes?

Tank of coolant loss is a worry, it is going somewhere.

 

First Rover wires are pure crap and never last long.
What do you mean your coolant pressure is fine, it can't be you are loosing lots of coolant, how was it tested?
When you say different things have been tested such as the tranny and ABS, who is testing them and how, sounds like you have a mechanic who is stringing you along.