Engine: 2.3 F/Inj B23E/B230F: Volvo 240 sputtering/wont accelerate

[MartinJepsen]

Hello guys, this is my first post here.
I have a 1987 Volvo 245 with a B230E engine, using the K-jetronic fuel injection system.
The car is displaying the following symptoms:

1. Accelerates fine through 1st gear, but seems to loose a slight bit of power in 2nd and 3rd gear.
2. Severe loss of power in 4th and 5th gear, along with engine shaking (as if putting it in too high gear), sputtering and the inability to maintain speed. Car will accelerate to 80 km/h in bad conditions (uphill for example) and 120 km/h in good conditions (wind from behind and going downhill).
3. When applying regular amounts of throttle, the symptoms are barely noticeable until going about 100 km/h.
4. Local mechanic says that the ignition timing is retarded, and that the engine seems to be knocking
5. Weird, rough idle when engine is cold

These are the things I have done so far:

1. Replaced spark plugs long before problem occured
2. Replaced fuel filter
3. Fuse for in-tank fuel pump was blown and the pump was barely drawing any amps, so i replaced every single fuse and the in-tank pump.
4. Problem didn't go away, so i swapped the main fuel pump and the fuel pressure accumulator for used units.

Does anyone know what the problem could be? My father is certain that the problem is not fuel related, but instead ignition related (he mentioned something about vaccum in the distributor).
Any help would be much appreciated!!