Interesting. I'm on a custom ROM... Second one.
But also have a Telkom SIM in slot two for data. Will try swap them around. Been like this for a year now though.
When I bought the phone, I was still on data package with CellC, my sim 1, it never wanted to go on Roaming on Vodacom, but almost never had problems with signal. Had one random reboot. Since my Telkom Mobile sim in slot 2 was never use for data then, it also never cause a problem. When my CellC expired and I switch to sim 2, Reboot all the time when trying to go on MTN 4g roaming (I assumed, because like adrianx said, if forced om 3g / h+ no reboot). Then I ported my CellC number to Telkom Mobile too and switch to sim one for data again, it works no problem. The telkom Mobile sim in sim one I kept on the default Telkom plan. Sim 2 is on there mobile broadband, so if they have great Telkom only data deals can switch to that sim, just disabled the data roaming. Hoop it works for you too.