How can I resolve my recurring ECU problems( restricted power output)?

Hi, I have a frontera 2.5TDS 1998 manual which has a power limit a 2000rpm (most noticable on a gradient. Drop below 2000 and the power returns. I believe that this is a safe mode of the ECU. Diagnostics indicate a number of codes but cannot isolate any as the culprit and when cleared and the car performs as expected for some time (it varies from a few weeks to 6 to eight months), only to return. Any ideas on how to resolve? Failing resolution is there any way of resetting the ECU to normal settings ( ie clear the codes with out the need to resort to a garage/diagnostic computer?