Diagnostics for VDC and associated HiRes RAM

[PyroferUK]

Hi, I have a C128 with intermittent problems.
It always works in C64 mode but C128 doesn't boot a lot of the time. I get a correct border and screen colour but random lines of characters in it.
When it boots it seems to work fine, I can load and run my old basic programs. The C128 diagnostic cart ran and said that both banks of ram were fine.

However, If I boot in 80 column mode and scroll the basic screen it crashes with random stuff over the screen. Pretty reliably almost every time after scrolling the same number of lines.

Oddly enough I loaded and ran the RISEN Obvlivion demo and that seemed to run fine. I am very confused.

Does anybody have code for testing the VDC and it's ram?

[motrucker]

You may already have fixed this, but I have one question - Does your 128 have 16Kb or 64Kb of VDC RAM?

[PyroferUK]

It HAD 16k, it now has 64k

Also, the VDC was duff. I replaced it and all is well.