Definitely read the codes. See the 700/900 FAQ on these pages for how-to.

Odd idling and running sounds like an amm on its way out. Air Mass Meter.

Also, the Idle Air Control controls idle speed. Sometimes they get gummed up and need a cleaning.

The usual way to check the AMM is to disconnect it (with ENGINE OFF) and see if it runs better. Never unplug or reconnect with ignition on. Another way is to get your hands on a spare AMM and keep it around. Whenever you suspect yours might be bad, swap in the spare and see if that fixes it.

Unfortunately it's an expensive part. Used is usually better than reconditioned, surprisingly. Fortunately Volvo used only a few types on the 240s so a visit to a junkyard might find you one. To see the type ID number, pull the AMM and see the last 3 digits of the part #. Yours (1989) should end in 016 or 001 according to the listing at fcpgroton.com. I suspect the standard tranny ones are 001 as the auto ones that I've seen are 016.
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Sven: '89 245 NA, 951 ECU, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors. Wifemobile '89 245 NA stock. 90 244 NA spare, runs.