Constant speed, revs all over

My TATA xenon 3.0 Dicor 2013 with 40 000km on the clock is now starting to give me problems. When idling along in low revs and low gears, there is a huge flat spot in response when one accelerates. The Rev counter fluctuates from 120 to 20 km/h whilst driving at a constant speed in 5th gear. When the Rev counter goes down, there is a loss of power and when it goes up there is an increase of power. I removed the tank and cleaned the strainer, I replaced the filters and also removed the accelerator pedal and inspected the electronic unit. I will post this all over the Web until I am satisfied with the correct diagnosis or feedback. I visited TATA in Nelspruit and the first thing I Was told was to pay R670.00 per hour to do diagnostics. Seems like everyone working at TATA is either not trained or do not know a single thing about the vehicles they work on or they just ignore the publics questions.