The engine may be running rich for some reason. Gas engines will happily burn more gas than they really need with few external symptoms. Pull one of the plugs and see if it looks black and sooty. A common problem that makes D-Jet cars run rich is a bad coolant sensor or a bad manifold pressure sensor. Get a multimeter and test these using data from this link : http://www.icbm.org/erkson/ttt/engine/fuel_injection/d-jet.html

As for the transmission woes - I'm guessing this is an auto tranny? If so take it somewhere and have it adjusted - things are slipping. I frankly have never had much luck with those cursed slush boxes - they make a Volvo sluggish and slow and they usually fail in someway after i drive them around for 3 or 4 months. Then again my parents put 250,000+ miles on their 71 145S wagon's auto tranny with no problems.

If its a manual tranny the clutch is slipping badly. After driving like that its probably toasted, but still possibly might be revived for a while with an adjustment. If not a clutch replacement is in your future.