Is it just sat on the tiddly little tilt lock?
Are you meaning the engine moving relative to the boat, or just generally bouncing up & down in harrmony with the boat?
See above for the latter. - and I would be very wary of lashing it direct engine to trailer - the boat & traier will move relative to each other - only a little, but enough to slacken any lashings - especuially if it boucnces like you are saying.
Also what are your tyre pressures? Think of your car - you would pump them up for a loaded journey & refduce the pressure for a "solo with no luggage". Same with the trailer tyres - the Max may not be the optimal.
if the engine is bouncing relative to the boat, you need to find a lump of wood and some woodcutting skills! What I did with my old Suz was found an old bit of 4X4 fence post, shaped one end to nestle nicely on the swivel bracket, the other end I drilled to take the tilt pin.
I then used 2 snap shackles and a 4:1 pulley with a cam cleat (a bit like a laser dinghy kiking strap) which I then clipped to the tilt pin, over the top of the leg (with a rag to stop the paint getting chewed) & pulled tight to stop it all moving.