Outboard Spontaneously trims self up??????

Hey all. I have weird problem with my outboard that I hope someone can shine some light on. I was cruising down the lake this afternoon and the outboard decides to spontaneously trim it self up. I was able to stop the upward motion as long as I held a finger on the down tilt switch on the throttle. I killed the power, reversed the Green/Blue tilt trim wires under the cowling and was able to get the motor back down. A few minutes later I happened again and again and again. I noticed that it occurs most when running at a higher speed (18 mph and above). Despite the wires being in the reversed position (up and down reversed) the motor still wants to trim up when underway, so I am presuming that the issue is not at the throttle handle or the switch on the side of the powerhead.
I sure there is a short in the wires somewhere but where. I am usually pretty good at tracking down issues like this but time is currently limited. Therefore I am checking to see if someone has suffered a similar situation.
The motor is a 1996 Mercury 60 HP ELPTO and is mounted on a center console Seaark.
Any ideas would be appreciated. Thanks.

Trim it down and disconnect all power to the trim to see if it's actually powering up or if it's drifting up due to a hydraulic problem.

Unfortunately it is powering up. When it goes up, it is very evident as the motor will rev when it goes up. After throttling down, the motor continues to trim up and the trim motor is audible. Thanks for the suggestion.

Only way that could happen is when 12v is energizing the motor when it ain't supposed to. You know that much so you need to look at places that it could come from. I'm not familiar your Mercury trim so what I'd do is wiggle and trace the wire harness and look for a chaff or short. Some motors energize with ground so it could be a short that's either. My johnson has up and down solenoids. You can switch them to see if the problem changes. You can chase the wire coming from the trim switch. Unplug it to see if that stops it. Take a meter with you and just chase it down. Could be simple to find or could take a while. You will find it with a little persistent hunting. If you don't have a meter or basic knowledge of electrical systems then this simple trim circuit would be a perfect time to dig in and see how easy and kinda fun electrical problems are to chase. Post a link to your merc trim wire diagram and we can sort right thru it.

I had a short minute to pop the cowling off this morning. I don't immediately see any chafing of the wires. There is a lot of oil that has accumulated under (which is a new issue) there and some moisture. That may be the problem. I am going to try to clean it up a little and let it dry out.

On my Ranger bass boat, I had a switch on the bow to control the trim. If that switch got wet it would do the same thing. I had to spray it down with silicone every once in a while to stop it.

I was able to trace the problem to a bad relay on the trim system. The up relay had developed a short some how and vibration would cause it to activate and trim up. $20 for a new relay and the problem is fixed.