Looks like the latest 2862 firmware 3.9.5_BT causes constant reboots.
Had to factory reset and tftp boot to older version to get router stable again.

The 3.9.5 BT firmware was extensively tested with lots of release candidates. It's a shame it didn't catch all of the issues though, can you get the output of "sys ver dbg" and report that to the technical support team?

Even though the router's been factory reset, I think it might still be recorded on the router.

OK I did just that and will see what they say

3.9.5 fails to load on 3.9.4.1 . Dithers for a minute then falls back with a small splash screen saying too many reboots.

Same here. Reverted to backup firmware when trying to upgrade to 3.9.5 due to number of reboots.

sys ver dbg
gives:

---

firmware exception debug info

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EPC=80ce207c, Cause=10800028, Status=1100fc03, BadAddr=84989c00, RetAddr=836f9474
exception id=10(RI: Reserved Instruction)
exception occured in IdleTask Timeout
task_id=147, SP=0x836f8f48(3176 bytes free, 5712 used bytes)
system tick =204236
CURRENT_VERSION : 3.9.5_RC9
get from address : 80004000
Dump CPU registers:

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$ 0: 00000000 fffffffe 00000000 00000000
$ 4: 00000001 00000000 00000000 00000641
$ 8: 00000000 83fb1e1c 00000000 00000000
$12: 58002000 836f9158 00000000 00000000
$16: 00000000 00000000 aaa7af32 00000000
$20: 00000000 83daec00 84989c00 00000001
$24: 00000000 00000000 00000000 00000000
$28: 81823e40 836f8f48 830c6d38 836f9474
List a possible caller functions:

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func( 0)=811ebf48
func( 1)=811f0000
func( 2)=80209ccc
func( 3)=8118408f
func( 4)=80bae0d4
func( 5)=80bdcda0
func( 6)=8079d488
func( 7)=8025a9a8
func( 8)=8074b86c
func( 9)=8074f554
List backtrace functions:

---

func( 0)=80ce207c

Upgraded ok here. Been stable for a day or so.