It might just be the shifter is loose on it's mount.
Lift the console or boot to expose the base of the gear lever. There is a large, 2" nut that holds the stick down into the mechanism. This sometimes works loose when selecting reverse and then it behaves as you have indicated.
You may need to loosen it off a bit more, work the stick until it behaves like normal, then tighten it down so it's snug.
Hope this is the issue, if not the Swedish Volvo P1800 club has manuals posted here;
Volvo P1800 Manuals, Handbooks,Volvo classic cars
Lift the console or boot to expose the base of the gear lever. There is a large, 2" nut that holds the stick down into the mechanism. This sometimes works loose when selecting reverse and then it behaves as you have indicated.
You may need to loosen it off a bit more, work the stick until it behaves like normal, then tighten it down so it's snug.
Hope this is the issue, if not the Swedish Volvo P1800 club has manuals posted here;
Volvo P1800 Manuals, Handbooks,Volvo classic cars