New Lifters - OM602 noise after highway run

[Gwalter]

Hoping to gain some ideas for a solution. I have a 1992 2.5L 300D with 254k miles. I've had it since 130k miles. I've read all around about lifters but mine act up after running at highway speeds. At highway speeds (3k rpm) things sound just fine. It's when I come off the highway that it is most disturbing.

Here is the situation:
Small tick developed in the last 6 months. It became worse (louder clacking type sound) thus I had all 10 lifters changed. They were really nice and quiet for two weeks after the repair. Then the small tick came back and it worsened just like before. I had the car examined again and it was determined that about 6 lifters needed replacing. So, that was done with the idea that some of these new lifters were "bad" . About 2 days later, small tick appears and it has worsened again. Here is the scenario going on:

If I drive the car around town from a cold start at low to moderate speed (normal around town driving) things are quiet. I would say sometimes I hear one of them making a tick. If I run on the highway and come off the ramp, it sounds like many of them making noise and it seems louder (maybe because it's multiple lifters doing this). If I then run low to moderate speeds after coming off the highway then things get pretty quiet or at least improved but the noise is not completely gone.

Mechanic cranked motor and observed oil galley was producing good oil flow with just the starter motor turning the engine. Fuel lines were loosened to see if it was injector noise. Nope, it's not injectors.

Oil pressure at cold idle is pegged at 3. At hot idle it is just a needle width above 1 (call it 1.1) Before engine reaches full hot, it's about 1.5 on the gauge. Note, when at hot idle the oil pressure needle bounces up and down from about 1.1 to 1.3 pretty rapidly.

I've faithfully kept oil and filters and the little o-rings changed at 5k miles. I use 5w-40 Mobil 1 Turbo Truck/Diesel. I've used Mobil Delvac 15-40. Pretty much no difference no matter what oil is used. The noise is disturbing.

Car doesn't use any oil or lose any coolant. Starts and runs great otherwise. Hoping for some ideas.