X2, what Diseasel said. Classic symptoms of no turbo boost, usually due to a plugged overboost valve (aka switch-over valve). Could also be due to failed turbo (can you spin by hand, access via front air intake accordion plenum), broken or disconnected boost line, busted ALDA.

Other possible causes you've got in your list (air filter, plugged fuel filters / tank screen, plugged exhaust system).

Make sure the brakes aren't dragging.

EGR failed wide open causes a massive power loss, accompanied by lots of smoke.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC