Hmm, all the PCINTs are same priority level for interrupts, so the first doesn't get interfered with? Maybe not:
The complete list of vectors is shown in ”Interrupts” on page 57. The list also determines the priority levels of the
different interrupts. The lower the address the higher is the priority level. RESET has the highest priority, and
next is INT0 – the External Interrupt Request 0. The Interrupt Vectors can be moved to the start of the Boot
Flash section by setting the IVSEL bit in the MCU Control Register (MCUCR). Refer to ”Interrupts” on page 57
for more information. The Reset Vector can also be moved to the start of the Boot Flash section by
programming the BOOTRST Fuse, see ”Boot Loader Support – Read-While-Write Self-Programming” on page
4 0x003 PCINT0 Pin Change Interrupt Request 0
5 0x004 PCINT1 Pin Change Interrupt Request 1
6 0x005 PCINT2 Pin Change Interrupt Request 2
So I guess put your more important software serial on PCINT1 pins vs PCINT2 or 3.
Or, review the software serial code, see if it messes with interrupts so tranfers don't end up with timing that could be jerked around.