In summary.
ZX Spectrum +2 (grey) displays typical RAM failure pattern on power on.
Diagnostics ROM indicates IC31 bit 1 faulty.
Removed IC31 and tested off board, and passed. RAM is good.
Socketed IC31 and replaced with a new 4164 but faster speed.
Problem still persists.
Other actions taken but still not fixed.
1. Removed, socketed and replaced Amstrad 40058 Multiplexer with ZX8401
2. Removed, ROM
3. Removed, socketed and replaced IC18 bit 1 RAM on second bank
4. Swapped Z80 out
5. Cleaned and reflowed sockets for Z80 and ULA
6. Checked continuity for RAM and Z80 control signals. No breaks found.
7. Scoped data and Z80 control lines, no obvious faults
Other symptoms
1. Diagnostics ROM reports stuck key which sometimes clears
2. There is no screen corruption when running the Diagnostics ROM
3. Occasionally the RAM test also reports that bit 4 has failed and has once or twice passed.
PCB is in good visual condition and no previous work has been undertaken.
Other things left to try (in no particular order just a desperate list of things I can think of)
1. Replace ULA.
2. Remove AY
3. Remove Joystick controller
4. Replace all RAM
Right
Beer