Sonar X3/Artist hanging notes on multiple vst-plugin-instances

The System: Sonar X3 (64Bit) on Windows 10(64Bit) with 16GB RAM.(Same issue before with Win 7 64Bit)
 
Note: All Midi Data we are talking about, come from an external Midi-interface.
 
Setting 1, that works fine: ONE instance of a soft synth. In my case Play by East West. I can have 16 Midi-Tracks with different Midi Channels and Route them to Play, where 16 corresponding different Voices will sound. All no problem. I can even record all 16 Midi tracks at once.
 
Setting 2, that goes wrong: As soon as I create a second instance of Play, that should pick up Midi Data from a second connected Midi Interface the trouble starts. Unpredictable, but quite soon after a few seconds a sound garbage will appear with several hanging notes.
 
To make it more obvious, lets create a very simple Setting in order to rule out/minimize a performance-related issue:
 

• 2 instances of Play
• Each instance contains only 1 performance/voice.
• 2 Midi Tracks that are set to route to the 2 instances of play. One to #1 and the second to #2 of Play
• Again we use Midi Data that come over the connected different Midi-interfaces(2 Ports)

Even with that simple setting no luck, after a afew seconds its over an hangs take place.
My first idea was to blame Play. No way, Kontakt has the same trouble. Even the little Microsoft GS wavetable toy cant survive.
When filling the Midi Tracks with Data INSIDE X3, lets say with the help of a standard Midi File, then there is no problem. It all plays fine.
Any ideas?
Greetings Ron
 

Are we talking about Settings->Midi->Playback and Recording-> where it says "prepare with xxx Milliseconds...?
As far as I understand the system this setting applies to Playback(!). But what I am actually doing is a recording through incoming Midi-Data.
Anyway, I played around there also with some values btw 100-500. No change.