Goto Settings > Safari > Advanced > Web Site Data. See how much data is being held by sites you've visited. Delete. Also try Clear Cookies and Data. May help.
Since iOS5, Safari (I have recently learnt) consumes large amounts of memory (RAM), and needs this tidy up operation. Take a look at Settings > General > About > Diagnostics & Usage > Diagnostics & Usage Data, and see if there are a lot of Low Memory diagnostics. If there are, check out which process is consuming most memory. If it's mostly Safari this could confirm your problem.
Alternatively, try a different browser. I downloaded Dolphin recently. Looks tidy.
You can't delete Diagnostic Data by the way (without a full reset). I'm about to raise a suggestion to Apple that they include a delete option in a future iOS.