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Without knowing more details, I'd have guessed the ECT block sensor or a bad electrical connection at the sensor or its grey connector block at the right strut tower brace. The sensor is presumably out of the picture now. Clean up the connections, coat them with a little dielectric grease and make sure the pins at the connector block are pushed together from the back side (there were problems with some early connectors). Dielectric grease should be in every Volvo owner's toolkit. Intermittent running problems are often electrical in nature caused by condensation or corrosion. You're lucky, people with '83-'87 Volvo's were additionally plagued by intermittent problems caused by deteriorated wiring harness insulation.
Almost all the starting problems I've had with my B234F were related to stalling when returning to idle when hot or immediately after catching with a warm start. This drove me nuts for years (I'm the original owner). The cure for this is simply a thorough throttle body cleaning including the flame trap and vacuum hoses plus maybe a proper re-adjustment of the throttle plate, throttle stop and linkages. As there aren't any street manuals for the B234F you may want one that describes this for the more common B230F (also try the 700-900 FAQ here). A thorough throttle body cleaning should probably be done at every other oil change or at the first sign of stalling.
Your backfiring immediately makes me think of bad timing, but short of a cam shaft seal leaking oil into the distributor I can't imagine what would likely be wrong (okay, maybe a faulty fuel pressure regulator leaking gas into its vacuum line). Remove the distributor cap and check for leaks. This leads me to my last point. The B234F doesn't get along well with valve carbon deposits. Your engine is low mileage and that often means city driving which in turn means mostly low rpm for a B234F+AW72L. Add the use of regular octane gas (87 R+M/2) and you'd easily get carbon build up. So, before you start throwing more parts at this problem I'd make sure I'm using a good quality gas like Chevron 92 or 94 (89 octane should be adequate once you get the problem under control). Then I'd run through a minimum of 4 tankfulls with a top quality injector cleaner additive at the recommended rate (Techron concentrate is good, but the professional quality BG44K and Toyota and GM branded top end cleaners are better). Add a few Italian tune-ups (a good long ride in the mountains at sustained high rpm) and I think you'll notice a major improvement including gas mileage.
-Dave '89 740 GLE B234F
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