I've been a lurker at this forum for quite sometime, finally getting frustrated enough to post...
I have a 97 850 2.4 with no turbo. A few months ago, the engine suddenly stuttered while driving at about 45 mph and the check engine light came on - I got codes, PO300 (random misfire) PO304, (cylinder 4 misfire) and PO302 (cylinder 2 misfire. I replaced the plugs, wires, cap, rotor, air filter and air filter with Volvo OEM parts and reset codes. Drove for a bit with some improvement, but still not 100%. The codes came back after a couple of trips to town. I checked for vacuum leaks and found some, ended up replacing the intake manifold gasket, the rubber elbows, the flame trap (housing and O-ring too), the vacuum tree O-ring, the oil trap and associated hoses. Again, a slight improvement to performance, but check engine light keeps coming back with same codes. I drove it anyway until I damaged the catalytic converter and had to replace that too.
Since then, I have cleaned all the electrical contacts to the various sensors, cleaned my MAF sensor with the special CRC spray cleaner, tested my crank position sensor, and throttle switch. I have cleaned the fuel injectors and swapped their positions to see if the misfires move, which they don't. I've taken apart the idle air valve and cleaned it to the point that I can hear the rattle when I shake it.

I am at a stage now where it idles OK, but with a slight shutter. When I put it into gear, it shudders badly and I have to ease the throttle to get it to move without stalling. Once I am up to 5-10mph, it seems to "kick in" and runs almost normally at higher speeds.

Some other clues - white "smoke" from exhaust, but only for a minute or two after start up, and the exhaust smells rich. If I pull the MAF sensor switch off while the engine is running, it doesn't seem to make any difference, but when I reconnect it the engine will stall. I read somewhere that is a sign that the sensor is bad, but I don't know. I hooked a vacuum gauge up and get a steady needle, although it seems a little low at 14-16 inches. I've been all over it with stethoscope and propane, trying to locate external vacuum leaks.

Does anyone have any suggestions to help narrow my focus?

Thanks