From those symptoms sound like stack overflow. Newer releases such as R5, when compiled with the KERNEL_USE_IDLE_FUNC flag set (as is the case in the default Arduino version of mark3cfg.h) will incur a stack size penalty relative to older releases. Try increasing the stack size, or disabling the KERNEL_USE_IDLE_FUNC feature and adding a dedicated idle thread.
Yes, Mark3 does support Arduino IDE -- the Arduino specific .zip release can be imported directly as a library. Note that support at this time is limited to AVR atmega328p and atmega1280/2560-based boards.
Threaded timer support code-complete, tested on...
Fix mark3c fake_types.h for notify object changes.