Sounds like the OD is slipping a little - its depends on a hydraulic pump to press a cone clutch onto the body to engage it. If yours is slipping its an indication more of a problem, not the wrong OD type. Volvo didn't specify an OD with barely enough capacity - I don't think you can overload a 'D' type with a stock B20 engine.

Check the oil level and possibly change it - I've heard lots of suggestions as to what to put in them (30 wt motor oil, ATF fluid, synthetic motor oil, synthetic tranny oil, Redline MTF, ect) but don't know personally. The solenoid could be getting flaky, or the pressure regulator (spring and ball) blocked open by crud or a little weak. Or it could just plain be a worn clutch or pump. You can hook up a pressure gauge somewhere on the OD and check the pressure, but I don't know the specs exactly. Something like ~40 psi off, 300 psi on???

In any case the earlier D types have a more tubular look to them - and the later J types have a squarer, chunkier look. I think the J-types have a solenoid on the left side and a plate that bolts to the bottom of the unit, while the D-type has a solenoid (with only one wire?) on the right side and a plate that bolts to the side.