Rodeo won't go to 4th gear
 

I have 2004 RA Rodeo - 4x4 Auto 3L turbo diesel.

Everything runs fine until driving at high speeds or at long period of time. After driving on the freeway, could be anywhere between 5-20 minutes, the revs go real high as if it's dropped back to 3rd gear and I can't reach the high speed that I should.

Car also turns off when trying to start the car but after a few a attempts I can usually start it.

I have taken it to a trans mechanic and they think it's not a transmission fault, it's possibly an engine or electrical fault.

Thanks