Checkout iFrame loading issues

For the most part the iFrame for Moneris Checkout works almost instantly, but we've been getting a few complaints every day for the past month or so from customers stuck on the blue moneris loading spinner with it never loading the iframe fields.  No matter what they do they cannot load the checkout payment iframe.  Then if they try it again in 10-30 minutes it works fine.  I haven't been able to experience this first-hand but it's been happening frequently enough for the last month that we can safely assume we are losing sales because of this issue.  Is there an issue with the Checkout system or would something on my end be causing the problem - generally once that blue loading spinner hits it's all about Moneris as far as I thought.

Thanks in advance for any help - I'll be giving them a call right away as well in hopes of answers!