It is possible that the new exchange rates code (for coinmarketcap and coingecko) is using more memory than it did before.
Unless someone shows me error messages, there is not much I can investigate further. But what I can do, is add some crash-detection code to the plugin. In the next patch the plugin will collect information in case of a crash, and will observe peak memory usage, and whether the cron tasks completed normally or not. So this might help with finding the root cause. I have already started working on this.
If you decide to downgrade, then yes, in this case, you would have to also downgrade to the previous version of the Exchange, since the latest one requires version 4.0.0.
with regards