2000 Chevrolet Cavalier White smoke

White smoke is typically a sign of burning coolant due to a leaking cylinder head gasket, but it is continuous, not just for a few seconds. GM vehicles normally do drip a lot of water from their tail pipes and there can be some steam too. Water vapor is one of the byproducts of a properly-working catalytic converter. Normally I'd tell you to ignore the water, but given the additional symptom of reduced power, you might consider having your mechanic perform a chemical test at the coolant reservoir. That is one test for a leaking head gasket.

You can also add a small bottle of dark purple dye to the coolant, then search a few days later with a black light. The dye will show up as a bright yellow stain. If you find that inside the tail pipe, it got there from the coolant and the head gasket is suspect.

Auto parts stores have the dye. They'll ask you what you're using it for. There's different bottles for engine oil, transmission fluid / power steering fluid, coolant, AC refrigerant, and I think there's one for brake fluid. The wrong one should not be used, especially in brake fluid.

Most auto parts stores that borrow or rent tools will have the black light too.

None of those things will cause white smoke, but spark plugs and wires are always a good investment, especially if they have a lot of miles on them. Some running problems can be solved by cleaning the mass air flow sensor and even the carbon from the throttle blade, but the symptoms will be considerably different than what you've described.

STP is a "viscosity index improver", meaning it makes the oil act like a higher viscosity oil. You can accomplish the same thing by switching to a higher viscosity oil at the next oil change. Oils today are really good and the additives do a much better job than they did 20 years ago, so there is little to be gained by using this type of chemical. If you DO use it, it's important to pour it in slowly while the engine is running and warmed up, otherwise it will form a big glob in the bottom of the oil pan and just sit there. STP will coat engine parts too so if any metal chips show up, they can get trapped in it instead of getting carried away to the filter. Those chips can be released later and potentially cause other problems.