Sounds serious!
This could be an indication to why the synth was cheap.
The manual explains:

O Internal RAM read error

Problem: The data of the internal memory has somehow been corrupted or destroyed.
Solution:Contact your nearest Roland service station

Suggest you do try a 'Factory reset':

Turn unit off 
Hold down lower right hand [8] key 
Turn unit on 
When "Initalize All Parameter" message is dispayed" press [ENTER] 
Press [WRITE] 

Hopefully this will set all the patches, performances and system back to how it was when it left the factory.
It is possible that the internal battery may need replacing, however warning is normally displayed as a 'Internal battery low' when powered up.

If it still shows this error message I think you may need to find a repair centre.
As you said in the original post, it's a great synth, would be better is it was fully functional.

Let us know how you get on.