2000 ML430, overheating in a strange way.

[CoyoteStarfish]

Just picked up the above vehicle for practically a song and a dance, knowing the previous owner was having issues with the cooling system and high temperatures.

So it goes like this, PO said it overheated, had a shop change the rad, fan clutch, water pump. Still overheating, so he left the car alone and unloaded it on me.

Now, I wanted to reproduce the overheat condition to get an idea of what is going on. Took it for a spirited drive around town for about 15 minutes and the temp never climed above 85-90c. With AC on.

Electric fans are operating, fan clutch sounds like it is engaging, no bubbles or pressure in the coolant system when idling cold. Cool beans.

Now I go to drive it home (about 1.5hr away), watching the temp gauge like a hawk and everything seems fine until about 45 minutes in the temp rises to about 110 suddenly, then drops to 95 within seconds, then back up to 120, then 100, then pegs at 120 and stays there. So I pull over, no discharge from the overflow. No tell-tale overheat "smell." No ticking of lifters. No post-overheat ticking or gurgling after shutdown.

Not sure what direction to go in for diagnosis. Any tips?