Too rich or vapor lock?

Folks, first post here so go easy on me. I am not an engine guy, so double easy on me!! 

Here's my issue. I am getting a small hesitation on the engine at times, almost like the engine burps (hesitates) for a fraction of a second. It typically happens at higher elevation with combination of high temperatures. I do get it at higher elevation and lower temperatures as well (68F) during climbing or cruise. At lower elevations (2,500ft MSL) I also get but not so frequently. There's no consistency on the "burps" it can happen one after the other, but I can also have 3 minutes without them then, a burp happens. 

Note the mechanic working on my engine is a Rotax specialist with 15+ years of Rotax experience where most of his time was spent at CPS.  

Here some engine information: 

912 ULS (1500TT)
Return line was installed with proper fittings (orifice etc) with hopes to prevent possible vapor lock
CHT Temps runs at: 140F
EGT Temps runs at: 1100F
OIL Temps runs at: 205F
Temperatures above are very typical and consistent, even on hot days

I have an electric, in flight adjustable prop which enables me to change pitch in flight.
I have a electric fuel pump that does not make difference on preventing the burps
Attached a quick snapshot of a cruise setting I was running at high altitude which was causing the engine to hesitate. I could adjust my prop to a higher RPM but chose to run at 5k
 
Mechanic and I have recently balanced the carbs and it runs very smooth. Also every component of the carbs was meticulously revised and everything on that department is within specification. We have also recently insulated every fuel fitting to prevent them for heating too much but that did not solve the issue. 

Next step is to look at carb needle assuming the engine is running too rich. Based on the EGT and CHT temps, and the case it "burps" at high altitudes, this could be an indication of engine running too rich.    

Curious as to you guys experiences and if I am heading to the right direction. If I missed any data, please let me know and I will update the post with any additional relevant info. 

Thanks

Alex