Replace the motor mounts. The US driver side motor mount usually fails first as the engine, when revved, pulls up on the US driver side mount. The exhaust passenger side mount gets squished, is sometime bathed in motor oil, like when replacing. the motor oil and fluid.
Dunno the state, yet get the diesel 240 mounts if you can. The failing or failed mounts allow an engine to move in ways the drive line does not like.
Next, verify the 3-5 mm of clutch cable free play at the throw out arm. As the clutch wears, the clutch cable slack is consumed. So you can expect what appears as slippi9ng clutch that is actually the clutch cable that is tight when you release the clutch pedal.
Also, do NOT maintain a depressed clutch pedal at stop lights, train RxR crossing waiting for a 4-mile long train that takes three or 15-20 minutes to pass with any manual gear box. Wears the throw-out bearing and does not good for the pilot bearing (replace clutch kit).
Press the clutch pedal ONLY when engaging gear and when removing from gear. Leave in neutral for those long stops. The owner manual states to use the parking or e-brake when well parking. I use first gear when parking on a flat. NEVER use reverse gear when parking.
Also, help to run through with grease or drip oil into the cable to keep it lubes and prevent fraying and breaking of the cable inside the sheath. (Not done this in some years, shame on me.)
I know the parking brake pads on any 240 by now, if factory original, require replacement with new parking brake pads. Some other bits may need replacement.
Else, with new motor AND transmission mounts, full or overfilled correct gear box fluid, proper clutch cable free play should resolve the symptoms you describe.
Also, when you replaced the gear box fluid, you did clean the magnet load plug (fill and / or drain plug, my 1992 has magnet loads in both fill and drain plugs).
The shift selector bushes, made of nylon, installed at the factory, can also fail. Replace these if lots of free play in the linkage.
If none of this works and the shift selector drops out of gear, you may have worn synchros or the gears themselves may be worn from high miles, incorrect or low lubricant, abuse like maintaining a pressed clutch for an enduring period, and more. Also, an old clutch kit with weak pressure plate springs may be a cause, yet doubtful. Worn bearings on or a worn input shaft may or may not also be a cause. Don't maintain a pressed clutch.
Don't do burnouts or horse around with 240/800/900 with M47. It is durable with gentle use and care and can last the life of the 240/700/900 in models without factory or added Turbo. Also, shift the M47 gently. Do not force or crash the gear selector. Ever drive a gear box without synchromesh? Like that. Know RPMs versus speed or as you up-shift feel the synchro engagement and like Zen or some published sections of Papas Kierkegaard and Heidegger you'll slide into gear. Raise the RPM when downshifting. A warmed gearbox may be easier to shift than a cold one.
Swapping to M46 is a pain. Not a drop-in. If you can get a good condition M90L, or newer, or one that has the third gear synchro welded from a global supplier as dumb dumb Volvo Cars never brought the to North America.
The M90 is a for real three shaft gear box, unlike M40-M47. I'm not sure, yet these are a direct drop in if the donor model has the silhouetting notch for the CPS. Yet scarcity is like that of hen and rooster's teeth. Last year for M90 in the rest of the world is 1998, when the final RWD Volvo was made. The M90 can take horsepower.
You may have options with a swap kit to Tremec T-5 with bell housing to fit that requires fabrication including drive shaft shortening and re-balance while preserving the two shaft drive shaft. Someone somewhere in North America may have a used kit to sell for 1989 non-turbo 240, yet caveats on a used gear box, like M46.
Here, on the Turbobricks:
M47 popping out of second gear
https://turbobricks.com/index.php?threads/m47-popping-out-of-second-gear.333611/
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