Mega Drive random issues

I have a mega drive 2 and it's got a few odd issues

Launching CD games from my mega cd is hit or miss, and when they do load they tend to freeze up

Sms mode works fine

Some MD games work fine but others freeze nearly as soon as they start, such as Marvel Land crashes during gameplay but the demo attract mode thing runs without issues

I loaded up Mega Doctor on my MED Pro and it didnt show any issues. I ran the z80 ram test and that passes.

What should I look at?