The 6 fet ones can get very hot (to hot to handle at times) and need to be in free air for cooling, if they are in a bag of any kind then there is no air circulation The ali case acts as a heat sink for the mosfets, a combo of slow speed and high power esp on hills causes the mosfets heat to rise which is why the case becomes a hot potato. You can add extra heat sinks to the case to try and help, add a small 5v fan or two, place it in free air or attach it to the bike frame and use the frame as a heat sink.
When mosfets reach their max temp they cut power until cooled (thermal cut off) and the bike is powerless it may take 5 -10 mins for them to cool sufficiently for power to flow again.
I do use 6 mosfet ones and have no issue on my commute/town workhorse, on my 700c Boardman ( more of a touring set up) I had issues and opted to use a 9 mosfet larger controller for better heat control.