CDTV Image Flickers on Certain Resolutions

[Astra Dan]

Hi, I have two CDTVs.
I recently brought them down from the loft to remind myself why I have two.
I think the display board died on the first, so I bought the other which DID work fine two years ago but doesn't now.
Anyway, I've carried out the display port short mod and now both work fine.
However, I've noticed there's an annoying jumping and flickering and a strange square pattern overlay appears on screen at certain times. It only does it on the start up screen and when using WB or WB-based apps, like Scorched Tanks.
I've been playing Turrican, Pinball Dreams (with the jumper pulled to boot in Kick 1.3) and Worms for hours and they're all fine, so it must be a resolution thing? Both units do the same.
I was going to buy a memory expansion for it to use it more, but not worth it in it's current state.

Any ideas I might try?
Many thanks.

[Astra Dan]

Hi thanks for your reply!
I actually read your same reply to another guy on  post in 2011. Both have been turned on for many hours and it doesn't seem to help. I can't see any blown or bloated caps, or any signs of leakage.
I guess I'll just risk recapping one of them to see if this helps.
I had to tear the warranty sticker on one to do the display board bypass, very sad!

[Astra Dan]

Hmm, my A1200 does it too, again only on Scorched Tanks.
I figure it must be my TV, but my A2000HD is fine. Weird!