Speedometer work on and off on a 1997 850 Volvo

use degreaser and water at tap pressure to wash the degreaser off
jet pressure puts water where it would not normally go

since they both work off the speedometer cable/electrical and the odometer still works this sounds like the speedometer head is bad and needs either repair or replacement.

Clogged up PCV System ---maybe

Too much crank case pressure

Clogged oil trap (PCV system, positive crank case ventilation). The hoses and oil trap need to be replaced (due to become brittle, dry rotted, and clogged) as well as cleaning out the sludge in the bottom of the engine block. The parts cost about $150 last I checked, and you will need to remove the intake manifold and spark plug wire cover to access the hoses and oil trap. On the bottom of the trap it connects to the engine block where you have to clean out the oil sludge. If you do not clean out the oil sludge it will still create back pressure through the oil cap and dip stick. I actually did an oil change at the same time, and drained the oil first and left the drain plug out while. I sprayed Sea Foam and compressed air down there to get the sludge out. I also used a coat hanger and brush to scrape/scrub it out. Worked good.

Check the engine breathers are clean,there is a small gauze filter in the breather hose, this will need cleaning or better replacing

Your PCV / Oil trap is likely clogged. I have had some luck running Mobile 1 Synthetic oil and half a can of seafoam in the engine oil to clean things up but it is likely that you will have to replace the pcv system.

It can if too much is used or you do not allow it to soak into the filter long enough. You can easily clean it from the mass air flow sensor with CRC, part number 5110.

Oil level
coolant level
Also it could be a O2 sensor gone bad very common

A bit of splash is normal from the slight backpressure created when the pistons go down in any engine, but might also be due to clogged PCV valve. Put the cap back on and pull the dipstick and see if you hear constant hsssssss from there, or if the oil splashing abates from the Fill hole.

As a rule, you are not supposed to open that with the engine running ;-) Very messy on a nice Volvo

My money is on the PCV valve (Positive Crankcase Ventilator ($3.00-20.00)

Make sure the pcv valve is working correctly and allowing the engine to breath