75 hp mercury optimax elpto power trim problem

[barefootpoppy]

I was having a problem with the power trim at full throttle to get up on plane. The motor drops right back down. I did replace the tilt trim assembly with a new one. The symptoms are exactly the same as the old assembly. When out on the water at throttle up to get up on plane the motor just drops right down. I can go about 5 miles per hour and the motor stays up on increase speeds the motor drops right down. The motor is a 2006 75 elpto optimax serial no. 1b244674. Hard to believe a brand new oem mercury trim assembly would be faulty and produce the exact same symptom as my old trim assembly. All three trim buttons work correctly. The one at the front of the boat, the one at the throttle control, and the one on the motor. I did check to make sure the manual release screw is tight and fill screw at full trim up for fluid level is full. I have the factory repair manual. Can't find any info on the trim sensor. Doubt it's that though since all electrical seems to work. Help?