extjs 4.1 grid hangs on loading - random

extjs 4.1 grid hangs on loading - random

I hope there is a solution for my issue. I'm using Extjs 4.1. I have a
viewport with a panel that allows to enter search parameters at the top
and 2 tabs at the botton. Both tabs have grids. The grids on both tabs are
very similar in the set up (not the data).
The grids support remote sorting and filtering and one also supports
grouping. Both grids have stores with autoload = false, take 2 extra
parameters (date and type), buffered = true and pageSize = 100.
Proxy type is rest.
Both grids are loaded at the same time when a search button in the top
panel is clicked. Search parameters are passed as extra parameters.
I set the ajax time out to 10 minutes.
Sometimes when I click the search button at the top, one of the grids is
stack with the Loading mask. Log file shows the controller is called and
returns results fine. Looks like when the json is parced - the grid
loading is halted. I won't be able to upgrade to a new version at this
point.
Any suggestions? May be how to detect when the load is hang and try again?
Thank you.