It definitely sounds like a broken spring. To verify it's the spring, try starting it on a block or something so all 4 wheels are off the ground. When you start it, or even just pull the starter, the wheels will spin if it's broken. If the wheels are on the ground, it will be VERY difficult to start because the clutch is engaging as you pull.
Refer to this tune and repair thread next - http://www.rcuniverse.com/forum/m_4207842/tm.htm t's the first topic on the thread.

Now, for the upgrades...
I have the SV as well and replaced the stock 16t CB with an HPI heavy duty (hardened steel) 18t CB for the Savage. Adding 2 more teeth greatly improved the top speed as well as kept plenty of torque, which this truck/engine has an over-abundance of. The 16t CB just winds out way too quickly. The HPI Savage CBs are direct swaps, no mods necessary.
I replaced my clutch springs with MIP replacements, but some folks here will recommend Traxxas replacements. Either ones will work fine, but it's hard to find the Traxxas spring alone without the shoes. They don't make a better compatible spur gear, but if you have your gear mesh set correctly, you need not worry. You will need to reset your mesh when you take the engine off. The best way is to stick a strip of notebook/printer paper between the spur and CB, push snug and tighten down the engine mounting screws, making sure to use blue LocTite on them. When you push the truck it should roll smoothly with no binding or loud clicking noises. It's a fine line between perfect and too loose/too tight, so be diligent in your adjusting. Read my thread I linked to at the end of this post. Very important info in there!
I also upgraded my stock wheels and tires with the HPI SS Monster Wheels and Pro-Line Masher 2000 tires. Super sweet setup, and these tires hook up like mad! Nothing stops my Volcano now...

Long story short:
Clutch Bell - http://www2.towerhobbies.com/cgi-bin...?&I=LXFKM8&P=7
Clutch Spring - http://www2.towerhobbies.com/cgi-bin...?&I=LXUR89&P=7
Wheels - http://www2.towerhobbies.com/cgi-bin...?&I=LXM875&P=7 (all 4 REARS for a 4X4)
Tires - http://www2.towerhobbies.com/cgi-bin...&I=LXUM93&P=ML

Here's the thread I started when I replaced my CB and spring - http://www.rcuniverse.com/forum/m_4982701/tm.htm
And heres the one on my wheels and tires - http://www.rcuniverse.com/forum/m_4865041/tm.htm