So, is the valve cover leak caused by the valve cover gasket or is it being helped along with too much crankcase pressure? Check the crankcase breather system for restrictions.

Increased crankcase pressure pushes the vacuum shut off diaphragm in the shut off direction. That might account for the erratic slowness issue.

Where does the crankcase pressure come from. Hot gasses going past the poisons, valve guides and from a leaking head gasket. Take the cap off of your coolant recovery tank and see if there is oil floating in it or if it smells like combustion gasses.

300 psi is not a great compression. I can't remember the lowest spec on that. You need to lest the compression for each cylinder and compare them for balance. If there is 2 adjacent cylinders lower than the rest that can be the symptom of a head gasket issue.

It could be you are having sticking piston ring issues. There are some threads on soaking the cylinders in something to free up the piston rings. (Compression test before and after helps you determine if the cylinder soak worked.)

Others have said using full synthetic oil and it takes like 300 miles of driving for it to free up the piston rings.

Not oil leak related. When the throttle linkage pivot on the fire wall goes bad throttle control can be erratic.

With the engine cold and off grab a hold of the throttle arm on the fuel injection pump and keep it from moving. Have someone sit in the car and slowly work the gas petal. Watch what happens throttle linkages and observe where there is paly in the linkages. In particular watch what happens to the throttle linkage pivot on the fire wall.

Each of the throttle linkage rods has a specified length it is supposed to be adjusted to.

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84 300D, 82 Volvo 244Gl Diesel