IBM 5160 oddball fault

I picked up a (known-faulty) 5160 a few weeks back, just because it had a keyboard with an intact decal, and the one on my otherwise-perfect system is missing. The machine came with a CGA card, which I've tested as working in my other 5160. Symptoms of the fault are one long beep and two short beeps at power-on, with 1024 characters of garbage displayed on-screen (typically either a solid block or a space, but with a handful of other random chars) - i.e. 12 rows of 80 and then a row of 64. Following this on screen, at location 13,64, the memory count occurs, and then it'll drop to ROM BASIC (no hard disk, and for the purposes of testing I've not bothered with a boot floppy), with the BASIC startup text starting at row 14 and all offset by 64 characters from the left margin. However, the function key reference on the bottom line of the display is OK, starting at position 25,1. I can issue BASIC commands, and each new line is offset by 64 characters, until I hit the bottom of the screen, at which point text appears at position 1 (but still offset from the top of the screen by 13 lines). Issuing a 'cls' doesn't remove the garbage from the display, and typing then resumes from location 13,64. Throughout all of this the cursor seems to appear where I'd expect it on the screen, however - I'm assuming it's done via hardware entirely within the CGA card. I've tried pulling all other cards, leaving just the CGA board, and the problem persists. One long and two short beeps seems to be 'video failure', but as mentioned I've verified that the card works in my other 5160. Does anyone have any ideas what might be going on? It doesn't quite seem like a memory fault - possibly some sort of address decoding error? It's almost like the video board is pulling display data from the wrong part of memory, but I'm not sure that makes sense given that the board has its own local RAM rather than relying on RAM on the system board. cheers Jules