I have the same problem on Disco3 2006 TDV6. Done 220,00 km. Goes to limp mode on highway on negotiating long inclines. Replaced fuel tank pump, air filter, cleaned transmission circular cable connector with electrical contact cleaner (this cured erratic transmission behavior, smooth operation returned) but limp mode still occurs. I hear a lot of groaning and hissing, but this is from the wife. I get the feeling it might be the accelerator control? It’s life-threatening to have to pull off the highway, turn off the ignition and restart then try and get back into the traffic stream.
