4x4 light flashing hilux sr5

hi everyone,
help required. I have a 2012 hilux diesel and the 4wd light flashes amber after about 30mins of driving in normal 2h conditions.
I can engage the 4h and then back to 2h and it goes off for another 30mins or so. it does however now and again take sometime for the green 4wd light to go off when i do this, unless i turn off the ignition.
All 2h,4h and 4L are working fine at the wheels when engaged.
thanks in advance for any help.
matt