I have an Isuzu Bighorn 1998 3.1D (engine RJC) with an automatic transmission (RMD). It behave perfectly until it drive up long hills (5km plus) after which it will not shift into 4th (overdrive). The cure is to let is idle for a couple of minutes and then it is fine. It has been like this for a year or so. My mechanic suggests it is loss of pressure on the 4th gear and wants to open the transmission and check the seals. anyone else had something similar? any advice?