I thought of something else after looking back at your original thread. I think that the stiffness is linked to fluid retention in some way (although it may be causal or both the consequence of overdoing–this I am not sure). I am a lot more stiff if I did too much the day before–standing a lot or walking a lot. It helps the stiffness to be moderate in activity (when possible). It also help to put my feet up when possible through the day or to lie down flat or with feet up late afternoon before finishing the day. I also now get up several times at night because of restlessness or discomfort and move a little to help from waking up so stiff. This helps mornings but is not so good for sleeping.
A few things that you could try would be the putting the legs up in the day when possible. Being careful about salt especially near the times of IV IgG might help (you get a salt load with IV IgG and water follows the salt).