It could be getting sluggish because of built up call history which needs to be culled or even totally deleted.
In addition to that, the cache could be cleared from many apps which may be contributing to the system slowing down, not to mention some apps being configured to update in the background, such as weather, syncing, etc.
If GPS is enabled, disable it until you really need exact location, such as when traveling.
Those things all will begin to slow a phone down after a while, sometimes just showing a symptom or two, as you report, then getting worse and spreading across the device's overall performance if not taken care of.
Go into applications > manage applications > menu > sort by size and then start clicking apps individually and clearing caches (not data).
Take a look at phone and message history to see how much build up there is; you may need to do some deleting.
Also, make sure that the proximity sensor, located as a tiny dot just to the left of the speaker, is clean; sometimes a dirty area there will cause the screen to slow down when using the phone from your ear to looking at the screen (the sensor darkens the screen to disable accidental dialing, etc while you're on the phone).
Then do a live battery pull for good measure.
