Hey Leah!
My name's Alex - I'm a Guru here with Shopify.
How often are you seeing this happen, every few days? Are you using a Business PayPal account or a Personal one? I'm glad that re-settting the gateway is working for you so far, but that's not the best thing over a long period of time. The next time you have an issue, email me at alex.missett@shopify.com and we'll see if we can figure out why this is happening.
Keep me posted :)
-Alex M.
Shopify Guru
alex.missett@shopify.com