2: Oil your cars properly. Too much or too little oil can prevent the car from running at it's optimum speed. I use the following methods. I am mostly running tjets, but I'm sure this can apply to most slot cars. 

 a) I use a heavy oil (like old red tjet oil) on the bottom of the chassis where the armature post sits in the chassis. Do not apply too much oil here, it's easy to get oil on the brushes, oily brushes will degrade the performance of the car A LOT. 
 b) I use a combination of a light oil like X2C and a heavy oil like Red Tjet oil, on the gears, I am pretty generous with the oil in this area, this helps quiet the chassis and reduces gear friction.
 c) I use a very light (x2c) oil where the axles go through the chassis, where the crown gear rubs against the chassis and where the rear gear post meets the chassis. 

If you've oiled the gears and the pinion and the axles and your car still squeals a bit, it's most likely the crown gear spot where it rubs against the chassis...

3: Make sure you have round tires! Many tires are not very round at all. Carefully pick tires that don't have flat spots on them. If your racing class allows, true the tires. I use the spin-sand method. I put the tires (even if they are fronts) on the rear hubs and hold the car just above a piece of fine sand paper. Then accelerate so the tires spin and lower them slowly to the sandpaper, this will round the tires out pretty evenly. Make sure your car is level or you'll have one tire a different diameter than another. Using a tire gauge or micrometer you should ensure the two tires are as close to exactly the same size as possible. Round front and rear tires help a lot in both acceleration and cornering.

4) Pickup shoe tension can cause hopping or stuttering. Hopping is especially notable when you are accelerating. The car will literally lift and hop as you floor it. (first check that your front tires are round) Generally the spring tension is too tight if this occurs. I take a small pair of diagonal cutters (dikes) and start by cutting off a half of a ring at a time. Each spring can be different and you want to have them both exactly the same. One way to tell if the springs are different is to carefully eye them and count the number of rings on each spring, then also note the height to ensure they are also stretched the same. Another way to tell is if you note a ridge developing in one of your pickup shoes, try cutting a half or full ring off of that spring and replacing the pickup shoe, or grinding it flat again with a dremel, and you should notice that with even properly adjusted pickup shoe springs, the car will run smoother, handle better, not hop and not wear the pickup shoes as much. The car rides on these more than anything, so this is probably one of the best ways to adjust your car's handles. Now, the other problem can be caused by cutting too much off of a spring, or just having a very compressed spring. If the car stutters, the spring tension may be to weak. Try taking the pickup springs and stretching them out a bit. If you can't get that to work, try replacing the pickup shoe springs with a brand new pair, with brand new pair, you may end up needing to start the cutting again if it hops of course...