Be suspicious of a bad coil producing a weak spark, possibly coupled with incorrectly gapped or bad plugs or bad contacts at the coil. See the following for appropriate testing of your Rex/Regina coil
https://volvoforums.com/forum/volvo-240-740-940-12/bendix-regina-coil-test-10640/
https://www.brickboard.com/FAQ/700-900/ElectricalIgnition.htm#ReginaCoilPackCleaning

Definitely check out timing belt alignment and condition. Have you got access to an okd fashioned timing light? That would be a quick and easy check of the timing. If it's around 10-12 deg BTDC you're probably good to go on timing belt alignment and the harmonic balancer not slipping. Othewise set the crank TDC using a probe in cylinder number one, remove the upper timing cover and check the cam sproket timing mark. The intermediate shaft sprocket for the oil pump doesn't matter, only when there's a side mounted distributor.

Just for the heck of it, double check the lower ignition wire from the coil is still fully seated in the disturbutor. Sometimes it's easy to think they're in all the way, but it's just the edges touching.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now