I quickly tested JetStrike and initially got the same as you using a standard A500 KS1.3 setup. The background graphics were messed up with corrupted blocks instead of the sky.

I then tested it with a standard A500+ setup and it worked. Just set the Quickstart menu to A500+ with basic non-expanded configuration and it should work perfectly.
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Cheers mate! Now all I need worry about is the landing gear....
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I just tested the game, and it worked fine with standard config A500 - 1.3ROM 512KB chip + 512KB slow RAM. Only got a guru meditation when loading new weapons, which I didn't get when using the A500+ non expanded config. Also took the time to make screenshots, so I'll post them soon.
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Were the sky backgrounds ok in game when you tested it with A500 KS1.3 setup? When I tested it with that config all of the backgrounds were really messed up, although the game did play OK and you just couldn't see anything flying other than your own plane. I never got a guru error with the A500 setup, just the messed up backgrounds.

The A500+ KS2.04 setup is definitely the configuration to use then.
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Backgrounds were just fine, as you'll see on the screenshots I made.
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