td4 power problem

Hi I've had the old thing 6 years just lately I've been getting a part time problem with the power under 2k rpm thought it was the maf so replaced it and fitted a ronbox2a aswell great bit of kit but still got the part time problem of low down power its not showing any fault codes or em lights its fine again if I leave it for a bit. its not smoking any more than any other td4 its had all the mods done not sure what to try next! Any ideas would be greatful as I've looked but nothing really the same! cheers. Ryan