Hi,
So you get different error messages and it connects sometimes. It is indeed odd.
The error, 429: Too Many Requests, can be seen when you hit the daily API rate limit. Or it can also be triggered by setting the wrong locale. Make sure you enter the correct keys, locale, and the Associates ID. If you type one of them incorrectly, it can cause an error.
I’d suggest you test your keys on ScratchPad. Make sure you don’t get an error there.
Also, if you are running multiple sites using the same API keys, it could be the reason to result in exceeding the rate limit. In that case, I’d suggest you try to reduce the number of API calls. One way to avoid reaching the API rate limit is to set a longer cache duration for each unit.
By the way, v4 (still in development) comes with a new unit type called Feed units. With them, you can import external unit data into another site so that it helps to reduce a number of API calls.