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Interesting problem and lots of ideas going on. My first reaction is that you've been throwing parts at the problem at a great rate and I hate to say it, that always bothers me a little. If a few educated parts swaps haven't made a difference then more diagnosis is in order. You've replaced/swapped half the fuel and ignition components, so (as Monty Python would say) now it's time for something completely different. My second reaction, without reading any of the other discussions, was that you've got some kind of fuel starvation problem. That's primarily based on your ability to feather the engine to higher rpms.
Not wanting to post and make a fuel of myself, I first did a little Googling and lo and behold over on Volvo-forums.com I found the following thread on 90 740 Gl Regina Acceleration Problems - Hesitation And 'stuttering' /'surging' Until 2000 Rpm. And what did the problem turn out to be? Ta da, the fuel pump! So my money is on that.
So how do you get over 2100 rpm with a worn pump? Good question. My rationale is based on the fact that fuel demand is not just a function of engine speed, it's also a function of richness which is increased by the ECU under high load or acceleration. Fuel demand might well be highest in the 1500-2500 range when the engine actually has to work to accelerate. Feathering it is basically just minimizing acceleration. As rpm increases, engine efficiency from output torque also comes into play. With the B234F in particular, the output torque curve is skewed to the right (higher revs) compared to the other red block engines so symptoms of fuel starvation would be aggravated.
The formal diagnosis for a worn pump would be measuring the current draw in amps and comparing that to a healthy or half-seized pump. I can't remember the exact figures and I've probably never seen them for your particular pump, but it should be something like 9-10 amps for a new pump and 14 amps for a badly worn pump (a few amps short of blowing the fuse or taking out the pump relay -hence the commonality of that problem with older 240s and their less than industrial strength relays). The informal diagnosis is how hot the fuel pump gets to the touch compared to a healthy one. Warning, they can get skin burning hot so don't grab one, use an IR thermometer or glove up and be careful.
Do not neglect the fuel filter. They are supposed to be regularly changed, but are often neglected. A plugging filter will place an added load on the fuel pump and hasten its demise. Same story goes for dead in-tank pre-pumps.
So that's my two-bits. Hope that helps.
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Dave -940's, prev 740/240/140/120 How time flies when you're having fun
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