performance issues in sprite kit with efficient code - sprite-kit

Question

I read all the posts I found about improving performance but my problem is a bit different.
I have a simple physics-based game. no super fancy stuff.
I´ve got max. 40 nodes on the screen and a few sklabelnodes.
my code is efficient - every node which moves outside the screen gets removed from his parent and Iv'e got just 4 physicBodys at once. textures are not to big - max 250x250 which are preloaded in an atlas.
the problem is that it works for a minute or so and then it starts to stutter for 4-5 sec and then it works fine again. that stuttering doesn't appear on a certain point. sometimes it does on startup and sometimes after a few minutes.
I don´t know what to do.
I load my ad´s on another thread and otherwise there isn't anything to load.
EDIT:
NO shapeNodes.
XCode 7 + iPhone 6plus (9.3.2)
#Alessandro Ornano
my update method only handles the background images. i already tried without bg´s. same problem. i read about performance issues in iOS9
do you know something about that?
-(void)update:(NSTimeInterval)currentTime{
//BG
if (self.background1.position.x >= self.size.width*1.5) {
self.background1.position = CGPointMake(self.background2.position.x - self.background1.size.width+1, self.size.height/2);
}
if (self.background2.position.x >= self.size.width*1.5) {
self.background2.position = CGPointMake(self.background1.position.x - self.background2.size.width+1, self.size.height/2);
}
//BG CLOUDS
if (self.backgroundClouds1.position.x >= self.size.width*1.5) {
self.backgroundClouds1.position = CGPointMake(self.backgroundClouds2.position.x - self.backgroundClouds1.size.width+1, self.size.height/2);
}
if (self.backgroundClouds2.position.x >= self.size.width*1.5) {
self.backgroundClouds2.position = CGPointMake(self.backgroundClouds1.position.x - self.backgroundClouds2.size.width+1, self.size.height/2);
}
}