The rear toggle/sight mod was almost certainly done over here, when who knows.
I'd leave the springs alone if it works fine as is.
There is little chance that someone fitted weaker springs, I don't know where they would get them anyway. Weak springs can cause as many problems as too strong ones- and each luger seems to have its own "quirks".
I'm with Ed, this hot ammo/US ammo thing is way over blown, not pun intended. US 9mm ammo is not that much different from European- and loadings from various manufacturers vary by around 100 fps - depending on which and whose load or data one believes.
Steel case ammo is not recommended due to the "obdurating" ring in the the original chambers.
The Germans themselves used steel ammo, and removed this chamber ring about the same time.
Check the stickys for more info.
Buy an original spring from "Lugerdoc" on this forum for replacing your hold open spring. First try bending it to increase the pressure a little to test if that is the problem- it may solve the problem for a while or permanently- just depends.
Keep reading, but take all you read with a couple grains of salt!
