I don't know whether this is a problem with the app/ hardware/firmware.

I pipe data to wireshark. After some messages around 30, CBT stops working. I need to reset MCU, restart packet logger, re pipe to wireshark and start wireshark to get it to log data.

This happens a few times till I get a continuous stream.

One thing I notice is the appearance in the debug tool of-
{"event":"version","name":"CANBus Triple","version":"0.7.1","memory":"839"}
{"event":"busdbg","name":"Bus 1","canctrl":"4","status":"0","error":"C0","errorText":"Receive Buffer 0 Overflow, Receive Buffer 1 Overflow, ","nextTxBuffer":"0"}
{"event":"busdbg","name":"Bus 2","canctrl":"4","status":"0","error":"0","errorText":"No Errors","nextTxBuffer":"0"}
{"event":"busdbg","name":"Bus 3","canctrl":"4","status":"0","error":"0","errorText":"No Errors","nextTxBuffer":"0"}
{"event":"version","name":"CANBus Triple","version":"0.7.1","memory":"839"}
{"event":"busdbg","name":"Bus 1","canctrl":"4","status":"0","error":"C0","errorText":"Receive Buffer 0 Overflow, Receive Buffer 1 Overflow, ","nextTxBuffer":"0"}
{"event":"busdbg","name":"Bus 2","canctrl":"4","status":"0","error":"0","errorText":"No Errors","nextTxBuffer":"0"}
{"event":"busdbg","name":"Bus 3","canctrl":"4","status":"0","error":"0","errorText":"No Errors","nextTxBuffer":"0"}
{"event":"version","name":"CANBus Triple","version":"0.7.1","memory":"839"}
{"event":"busdbg","name":"Bus 1","canctrl":"4","status":"0","error":"C0","errorText":"Receive Buffer 0 Overflow, Receive Buffer 1 Overflow, ","nextTxBuffer":"0"}
{"event":"busdbg","name":"Bus 2","canctrl":"4","status":"0","error":"0","errorText":"No Errors","nextTxBuffer":"0"}
{"event":"busdbg","name":"Bus 3","canctrl":"4","status":"0","error":"0","errorText":"No Errors","nextTxBuffer":"0"}
{"event":"version","name":"CANBus Triple","version":"0.7.1","memory":"839"}
{"event":"busdbg","name":"Bus 1","canctrl":"4","status":"0","error":"C0","errorText":"Receive Buffer 0 Overflow, Receive Buffer 1 Overflow, ","nextTxBuffer":"0"}
{"event":"busdbg","name":"Bus 2","canctrl":"4","status":"0","error":"0","errorText":"No Errors","nextTxBuffer":"0"}
{"event":"busdbg","name":"Bus 3","canctrl":"4","status":"0","error":"0","errorText":"No Errors","nextTxBuffer":"0"}
{"event":"version","name":"CANBus Triple","version":"0.7.1","memory":"839"}
{"event":"busdbg","name":"Bus 1","canctrl":"4","status":"0","error":"C0","errorText":"Receive Buffer 0 Overflow, Receive Buffer 1 Overflow, ","nextTxBuffer":"0"}
{"event":"busdbg","name":"Bus 2","canctrl":"4","status":"0","error":"0","errorText":"No Errors","nextTxBuffer":"0"}
{"event":"busdbg","name":"Bus 3","canctrl":"4","status":"0","error":"0","errorText":"No Errors","nextTxBuffer":"0"}
{"event":"version","name":"CANBus Triple","version":"0.7.1","memory":"839"}
{"event":"busdbg","name":"Bus 1","canctrl":"4","status":"0","error":"C0","errorText":"Receive Buffer 0 Overflow, Receive Buffer 1 Overflow, ","nextTxBuffer":"0"}
{"event":"busdbg","name":"Bus 2","canctrl":"4","status":"0","error":"0","errorText":"No Errors","nextTxBuffer":"0"}
{"event":"busdbg","name":"Bus 3","canctrl":"4","status":"0","error":"0","errorText":"No Errors","nextTxBuffer":"0"}
{"event":"version","name":"CANBus Triple","version":"0.7.1","memory":"839"}

repeatedly. So thats the cause, but what does it mean?

In your case, enabling MCP2515 buffer rollover could solve the issue.
If rollover is already enabled, see the bottom of this post.
Line 5 is where buffer rollover is enabled in this code.

1] CANBus1.begin();
2] CANBus1.setClkPre(1);
3] CANBus1.baudConfig(500); // <-- this might be different for you
4] CANBus1.setRxInt(true);
5] CANBus1.bitModify(RXB0CTRL, 0x04, 0x04); // Set buffer rollover enabled
6] CANBus1.bitModify(CNF2, 0x20, 0x20); // Enable wake-up filter
7] CANBus1.setMode(NORMAL);

Do the same thing on bus 2 and 3 if you use them.

If rollover is already enabled, the bus you are trying to capture might be CAN 2.0b (29-bit extended frames). If you are using the stock firmware, you need to add support for 29-bit mode. Also, make sure the baud rate is correct for the bus.