Losing power (again)

I have seen lots of treads about losing power, black smoke etc but not one that is the same problem I'm getting with my Freelander 2, 2008, 42000miles. I drive regular long journeys and occasionally power goes and I cannot get over 2500 revs. Down hill/level its not too bad but uphill it drops to 30 mph. Taken it into my local garage having nursed it 140 miles (!) and he plugged in the computer diagnostic kit and within seconds the problem seem solved. After the 3rd time it happened he researched the problem on line and decided it was dirt in the transmission fluid causing a valve/switch to jam (you can tell I am no tekkie!). Spent lots on having the transmission fluid changed, system cleaned etc only to have the same problem 2 months later! I love the Freelander but I need a reliable car. Any bright ideas?