will not stay in high gears on 1996 Toyota 4Runner

you can floor it and it goes threw all the gears then when you let up (75 mph) it is no longer in gear and will only drive in low you can stop and floor it but when you let up it will not stay in the high gears it has new torque converter.

Asked by for the 1996 Toyota 4Runner
seek a free diag and est from aamco then shop prices. do you hvae any trans codes?