does the 1460 have the same engine as the 915s 404 or 436 I had a problem with a 915 @ startup cool had good pressure as it warmed up pressure would drop found on the side of engine block large nut head this was an oil bypass, found it had something in it holding it partially open put a new one in an problem solved! good luck!
If idle pressure is normal even after its hot. Sounds like the oil pickup screen is plugged. Cant suck enough oil at high rpm to pump it. I also agree that a damaged oil bypass valve could do that. Either way replace the pump and screen. If the pressure is low at idle also when hot it could be a bad pump or a worn bearing bypassing pressure. Even with a bad pump or bearing the pressure should still increase some when accelerating. I would lean more toward a plugged screen.
If you got a 66 series tractor or 86 series tractor around the yard borrow the oil relief valve(big nut near oil filters) and try it in your combine motor. Keep your mechanical gauged hooked up and compare. This would be easy check before you go digging into the oil pump and suction screen Good luck
if it will idle with oil pressure but will lose preasure when motor gets warm and at hi rpm then you got some junk on the bottom of your oil pick up tube. i would go ahead put a new oil pump and screen on while you got the pan off just in case.
If the bypass checks out OK, pull the engine/oil pan and check baffle spotwelded in the oil pan, baffle breaks out and lays flat in the oil pan. At Idle works OK, HI rpm the suction tube will pickup baffle in oil pan and cutoff/restrict oil flow. Have personally seen this in more than one 1400 series.
If psi has been at near zero at any point you will need to freshen up the bottom end now as brgs are likely damaged due to low oil and could spin one sooner than later.
Last year someone changed the oil in our older 1480 and put way too much oil in the crankcase and when it got up to operating temp the oil pressure really dropped off i checked the dipstick and it was 2-3" adove full . Drained some oil out and then pressure was good i guess the crank was hitting the oil and foaming it up .