I did success with MySBootloader 1.3 beta 3 (https://github.com/mysensors/MySensorsBootloaderRF24 )
It works both arduino and OTA firmware upgrade however it takes longer to boot up (means switch to firmware) about 3-5seconds.
I'm using MYSensors 2.1.0 and MYController (http://www.mycontroller.org) on arduino pro mini 8Mhz 3.3V.
How many sensor you try at the same time?
From memory When I do firmware update for 3-4 sensors, I have to change "Tx message processing delay" to 300ms.
If it's shorter than 300ms, keep rebooting some stage.
I didn't use MYScontroller but probably it has similar setting.