99 burban wont shift under hard acceleration

Hey guys, Im beating my head against the wall... 99 burban Z71 shifts fine unless you are in the throttle. no codes are set or stored and the timing is correct. ckp and cmp both check out. No trans codes are on ether. when under hard acceleration you have to let up so it will shift to next gear. No it does not shift hard. Fuel pressure is 55psi at idle and bleeds down to 40 in about five min after shutting it off. Changed the fuel filter and still has same pressure readings. Does the low pressure have any thing to do with the shifting issue? Where do i go from here? Please help.