100 HP Mercury tilt/trim information

[SHADOW]

I have a 1988 Merc 100 4 cyl and it powers up all the way, but when I lower it with the trim switch, the motor runs fine, but it sorta drifts down instead of powering down to the two trim rams and then stops and won't go any further. The motor still runs but I can't even push it down the rest of the way into the trim rams. I can bleed it down manually. When I tilt it up, the motor runs for a few seconds and then the main single tilt ram takes over and raises the motor fine, all the way up. I have never had a power tilt before but I assumed it should power it's self both up and down. Any Ideas??