E300 Diesel 1996 poor mileage
 

I got this car about a year ago and lately it developed some issues. It has been driven by my wife so I can tell when those thinks occurred.
1 car is sluggish when warm
2transmission shift kind of slow sometimes is slipping 2-3 and 3-4.( on hard acceleration it shifts nice)
3The fuel consumption dropped to 20-24 mpg

What I did:
New filters and all fuel lines and o rings.(it was loosing diesel like 10oz a day)
New air filter
New K40 relay
Change trany oil with MB fluid
Change the engine oil with MB Mobil 1 ESP formula M

No improvement after all those thinks. Except that there are no more leaks.
I forgot to tell you that there are no codes or engine light on.
What else should I try?

Clean the intake manifold, including the flaps. The later turbos don't have them, but the 96-97 have flaps in the intake that are quite important.

Checked your tyre pressures lately?