I have exactly those problems. iMac Core i7 OSX, Delta with Marlin firmware (also tried latest 1.1 RC7 marling firmware). Also v1.5 had this problem. I hoped that with 1.6 the problem would be fixed. I want to use mattercontrol because of it's features but printing over USB is terrible. Always stopping and giving blobs in the print.The cause: Mattercontrol is (like other apps) reading the temps of the bed and nozzle during printing. Every time the temps are read the printer stops for 0.5 seconds leaving a blob on my print.Exporting from Mattercontrol and printing the gcode using pronterface or any other 3d printing app (which also read the temps) results in a perfect print. Somehow the serial communication from Mattercontrol on a Mac is slow (MONO layer).I dont like Cura but it does the job without blobs. THIS IS THE ONLY 3D PRINTING APPLICATION WITH THIS PROBLEM!So please Mattercontrol team fix this problem.