If I had to guess Id say it is the seals inside the valve body of the tilt/trim motor. The valve body is the bottom part where all the lines screw into. There are rubber seals in there that fail.
There is a sticky on the front page of these forums that shows how to remove the valve body and replace the seals. I had the same problem and the repair fixed it for 3 years now.
When you do the repair you should replace the oil in the reservoir. Suck out all the old stuff as much as you can and replace.
There are posts on here that explain how to do it if you search for it. You can add too much oil if you do it wrong and push oil up into the motor so make sure you refill properly. Good luck.
