What are the server configurations (hardware, and SP Services running on them).
You could be getting a lot of traffic, or if your running services on there like search or excel, they could be tying up your memory.
You can review the monitoring guide and start up some counters to watch what is happening
If you have SCOM in your environment you could setup the counters there as well. Additionally Idera has a performance monitoring tool that captures some basic metrics
http://www.idera.com/Products/Free-Tools/SharePoint-perf-monitor/ Their full product offers a lot more of course.
Once you have identified the offender you can decide how to proceed to alleviate the issue (add memory, move application, new server, etc). However if are resource poor, you may need to adjust some settings for the amount of memory the App Pool's can use, in addition the native SP throttling.
Links to review regarding AppPools: