I'll run down a list of likely suspects that I see often on my own and friends Volvos when this occurs.

1. Dirty electrodes on the underside of the distributor cap. The carbon builds up as the electrodes wear down and the carbon absorbs moisture. This causes the impedance to go up and misfires occur. Very common after a rain storm or in conditions of high humidity, especially in the fall with lower dew points at night.

2. Spark Plugs have the removable ball nut on their attachment point. A very common issue that I see are that 90% of the spark plugs available for our Volovs have the stupid, removable ball that the boot connector clips onto. These are notorious for loosening up over time and cause sporadic misses and the occasional backfire if the path of least resistance is through another electrode in the distributor and to another spark plug. This issue is more of a problem than dirty spark plugs. Also, the official Volvo plugs are worth every penny, they are impedance matched for the ignition system and resist fouling better than any aftermarket plug I've found. Lots of research has been done by Volvo and IPD and this is not a wheel you need to re-invent. Spend a few extra dollars on the Volvo plugs and reap the benefits that rest of us enjoy.

3. Ignition wires. If they are touching each other or touching a metal object, expect that you will have misses. Spark plugs need to have 1/8" or greater spacing between them and that can be accomplished with split sheath tubing (GM does this on there cars) or plug wire holders and separators, or combinations of both. Also note that most aftermarket plugs wires are garbage and either come apart easily, have poor insulation, or both. The factory equipped sets of plug wires are Bougicourd Type F and they will outlast the car if you don't abuse them. My wife's 1993 240 Classic with +400,000 miles on it is still using the original plug wires, and so is my 780 coupe. (Type F is the heat rating, avoid the C ones, and D is acceptable if it's the only one you can get your hands on.)

4. Small chance but worth mentioning: Some of the ignition coils have the spade connectors really close to the boot. I have seen other faults elsewhere in the ignition system cause high impedance and the spark arcs from the boot at the coil to the ground connection at the coil.

5. Small chance but worth mentioning: I have seen damage to the tone ring on the flex plate that causes drop outs in the signal for the crank position sensor. Example, a tow truck driver puts a recovery hook through the window at the bottom of the bell housing and makes a dent in the tone ring. At idle, the ignition behaves fine. As RPMs increase, the dent goes by too fast for a proper signal to be generated and it looks like the double blank at 45 degrees BTDC that resets the ignition computer to begin a new rotation cycle. This is probably not your problem since your issue is when you accelerate, rather than dropping cylinders above certain RPMs.

6. Check your accordion tube for cracks that are caused by rubbing against the inner fender wall. This is a very common issue. You can replace the hose or tape the old one with electrical tape or PVC wrap tape (use 5 mil thickness).

God bless and let us know how it works out.
Fitz Fitzgerald

P.S. This is my first time back to post a reply since 2014. Sorry for being gone so long, it's good to be back.