To ensure that your 1999 Nissan Frontier is running smoothly, there are a few steps you can take:

1. Check the battery: Start by inspecting the battery terminals for any corrosion or loose connections. Clean them if necessary, and make sure the battery has enough charge. If the battery is old or weak, consider replacing it.

2. Check the fuel system: Begin by checking the fuel level in the tank. If it's low, fill it up with the appropriate fuel grade recommended by your vehicle's manual. Next, inspect the fuel pump and fuel filter. A clogged fuel filter or a faulty fuel pump can cause issues. If either component is malfunctioning, they might need to be replaced.

3. Inspect the ignition system: Check the condition of the spark plugs and ignition wires. Worn out spark plugs can cause misfires and poor engine performance, so replace them if necessary. Also, inspect the distributor cap and rotor (if equipped) for any signs of wear or damage. If needed, replace these components as well.

4. Check the engine oil: Make sure the engine oil level is at the proper level and change it if it hasn't been changed recently. Refer to your vehicle's manual for the correct type and viscosity of oil.

5. Check the air intake system: Ensure that the air filter is clean and free from any debris. A clogged air filter can restrict airflow to the engine, negatively impacting performance. Clean or replace the air filter as needed.

6. Inspect the cooling system: Check the coolant level in the radiator and the coolant reservoir. If the coolant level is low, top it up with the appropriate coolant mixture. Monitor the coolant level regularly to ensure your engine stays properly cooled.

7. Perform a thorough visual inspection: Look for any visible leaks, damaged hoses, loose belts, or worn-out components. Address any issues you find promptly.

These steps should help to get your 1999 Nissan Frontier running smoothly. However, if you encounter any persistent issues or if you lack knowledge of car mechanics, it's always best to consult a professional mechanic for a more accurate diagnosis and resolution.