Thanks all for the guidance, and especially for pointing me in the direction of the auxiliary air valve. I went to clean it out, and in so doing noticed that the S-hose that connects from the top of the AAV to the intake manifold had become soft with age. It developed a crimp that severely reduced air flow. This was an aftermarket hose. I replaced with a genuine Volvo hose, which is of much thicker and rigid construction, and all seems much better now. Always gratifying and fun to receive a bit of outside knowledge to help explore the quirks of a 40-year-old car !!