I have a '96 965 that was developing the same problem. I called a Volvo dealer and spoke to their service manager who told me it was probably a bad intake manifold gasket, most likely #1 cyl. I took it to a very experienced independant Volvo mechanic who said the same thing and replaced it. This cured the problem! While you're in there you should probably change the crankcase breather box which is buried under the intake manifold, it's not expensive, and most likely needs replacement if it hasn't been.