New discoveries by myself: the stress test was not stopped by the IPS function, actually it has been stopped by the IOS firewall's maximum TCP incomplete TCP sessions to a host" function and access to the host was banned for 10 minutes completely (info found in syslog). I adjusted its settings as required and further testing will be done later.
In IPS I defined an IPS filter to releive required source IP address for testing, excempted from check.
If anyone feels to add something, welcome.