Because the temperature is getting warmer, it may just be that your engine coolant temp sensor has gone south. That will make the ECU think it is cold outside and inject fuel for longer time periods than if the air was warm.
I doubt it is the injectors, but a bottle of injector cleaner won't hurt anything.
How about a tune up? New plugs, wires, distributer cap and rotor. Keep your happy for $100.

Klaus
--
(V♂LV♂s 1995 854T, 1998 V70R)

How to pull codes on '95 and older 850's with DTC boxes under the hood.

Gather Codes;
Ignition off
Stick the flylead attached to the cover into A2 socket
Ignition on
press the button next to the led once
Codes appear as blinks, copy down the code onto paper.
Press button again after code displayed to see next code.
After all codes displayed, codes repeat.
(ie continue copying down codes until first code repeats.)
1-1-1 means nothing's there (hopefully, all OK).
Switch off ignition.
Just because a code is there doesn't mean that it is a 100% certain problem for your car.
Electric spikes etc may occur giving false readings on the diagnostic system.
Reset the system and then see if the error codes reappear.
Fault codes can't be deleted until you have read them all at least once.

Reset system;
Ignition off
Stick the flylead attached to the cover into A2 socket
Ignition on
press the button next to the led once for about five seconds
release
red LED will light
when lit, press the button next to the led again for about five seconds
red LED goes out
Ignition off, keep the flylead attached to the cover into A2 socket
Ignition on
press button once for a second, ensure codes are cleared
or write down code(s) repeat code gathering procedure.


http://au.geocities.com/ozbrick850/computercodesvolvo.html#instructions










Mike
'95 850T

The code 300 is just misfires on more than one cylinder. It would coincide with the 303 and 304 codes. Have you read the DTC codes from the DTC boxes under the hood? They may offer more information. How old are your ignition wires? A new set may solve your issues. But get any DTC codes discovered from the DTC boxes first. These OBDII P0XX codes you've discovered should be available from the A2 socket along with some other codes possibly shedding more light. They'll display as 4-4-3, 4-4-4, or 5-5-3, 5-5-4 codes. Good luck.






Mike
'95 850T