I picked up an NES at my local vintage game shop. It was "tested

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I picked up an NES at my local vintage game shop. It was "tested and working fine", but when I got it home it didn't play anything other than Mach Rider. I've spent the last few weeks fiddling with it but I can't get it to work better than pic related. The visuals and physics are all fucked, and games crash within five minutes. Even if they start up fine they end up like this. It won't even play Mach Rider anymore.
All of my games work on a reproduction console.
It works with composite cables, but won't even turn on with the RF cable.
Most of the time it will blink once and shut off. (Not the repetitive blinking of the lockout chip.)
Here's a list of stuff I've done so far:
>Disabled lockout-chip
>Replaced pin-connector
>Cleaned connections
>Cleaned games
I can't find much on what to do next. The gentleman at the shop says the board must be fried, and that I should just get a new NES. I don't wanna get burned again though. Thoughts?
If I should buy a new system, how can I avoid getting fucked?