87-300D Turbo (603.96) (185K miles. owned since 10/96)

Over the last year or so, the thrust so to speak has been dwindling on the high end, I could drive 100mph+ sustained or pass with rocket like performance with no trouble, and as time has past over the last 12 months or so, I notice a "missing" like behavior early on during high load passing where acceleration was very brisk, and then I felt a missing like hesitation, though very slight as rpm and load climbed. This has given way to deteriorating performance on the high to midrange now, were 65 mph is a chore.

Interesting symptoms,

If you hold the throttle to the floor, it will go to the governor stop at 5250 rpm for about 3-5 seconds and decrease to around 3500-rpm and thats it. The colder the engine, the quicker the fade away is. Initial breakaway is brisk, but gives way to an underpowered situation.

I have boost. So far tested items inclued; system vacuum tight at about 20 inches, the EGR removed and checked, air recirculation valve on the turbo checked, turbo spins free, the mass airflow door hinges freely against it's spring, both switch/change over valves (Y29 & Y30 functionally tested and swapped) over boost switch checks open and has no effect on performance if it is connected or disconnected, as i have no way of testing it's operating pressure and with it disconnect, this would presume normal function. All pressure lines tested tight with no drop in pressure, some rubber connections replaced.

So, I have three guesses left, first one:

Has anyone had a main fuel filter (spin-on) gradually clog as it seems that given enough "recovery", initial throttle responses is normal and gives way to the underpowered situation? The clear filter has good flow through it. I would have changed it already but I'm out of stock and World Pac is closed till the 26th.

Along these lines, would a tired primer/lift/lowpressure fuel pump exhibit these symptoms?

Lastly, does the ALDA ever fail/stick/require a once over, etc? I am reluctant to mutts with this as it was working normally and as I understand it's function, it provides reference infromation and it works or it doesn't. Or does it???

Feedback would be really appreciated, I feel like Doc and Johnny on the tonight show with stump the band as I have beaten this to death and I'm still missing the fix.
-Mark

Sorry Brian, good question the trap has been gone since 98 and I have the new catalyst style installed parts free of charge by MB along with a new turbo as well. The head failed at 103K, in '97 trap still installed, overhauled replacement installed and followed by the complete exhaust and turbo one year later. Post exhaust replacement, performance was perfect until 12 months back as i mention above.