I'm using this little dev board
http://www.aliexpress.com/item/ESP8266- ... 33472.html
The ESP device is identified as ESP8266MOD and the vendor is AI-THINKER.
From what I can find, the flash size is 4Mbits - but it's hard to be sure because both the ESP chip and the flash chip are enclosed in the metal can.
A previous flash of your BASIC with the 4M selection worked fine. Having reloaded the device I'm still seeing this issue.
Wondering if there is something corrupting the SPIFFS filesystem ... perhaps my board is toasted?