Torsten,

The things that could potentially cause the IPS to go into bypass which could in turn potentially cause the ASA's to failover are as follows:

Configuration Changes

Auto Updates

Global Correlation Updates (5 minute intervals)

Anomaly Detection

As a test you might want to turn off global correlation and anomaly detection and see if the issue persists. Below are the instructions to turn these options off:

Next is an example by using CLI to turn GC and AD off:   
   
# -------------- To turn off global correlation inspection   
   
sensor-43# config t   
sensor-43(config)# service global-correlation    
    
sensor-43(config-glo)# global-correlation-inspection off   
    
sensor-43(config-glo)# exit   
Apply Changes?[yes]: yes   
    
    
#-------------- To turn off network-participation   
    
sensor-43(config)# service global-correlation    
sensor-43(config-glo)# network-participation off   
sensor-43(config-glo)# exit   
Apply Changes?[yes]: yes   
    
#-------------- To turn off AD    
    
sensor-43(config)# service analysis-engine    
sensor-43(config-ana)#virtual-sensor 0   
sensor-43(config-ana-vir)# anomaly-detection    
sensor-43(config-ana-vir-ano)#operational-mode inactive   
sensor-43(config-ana-vir-ano)# exit   
sensor-43(config-ana-vir)# exit   
sensor-43(config-ana)# exit  

If the issue still persists after ensuring that all the above is off you might want to gather a show tech output. In the show tech output search for the word "main.log" and start searching via the time stamp for any messages around 2:00pm. Also what is your bypass mode set to (auto,on,or off)?

I hope the above helps!

Thanks,

Justin T.