SharePoint Online? Or on-prem? It may be you could set some query fields to help with this, but I wouldn't expect a query to a list with 400 items to cause this....does another one of your data connections return a greater amount of data? You may want to narrow down which one causes the issue by setting all of them to not run on load, then temporarily put buttons on your form to execute each query....Or, if you are on-prem, your SharePoint admin could also check the ULS logs.
Hilary Stoupa
