make sure it is the exact spark plugs.. Make sure the put the timing belt in correctly. Also, possible you have some severe valve damage. Intake or exhaust valves could have seized up... I had 16 valves on my vehicle replaced, the cylinder head was redone, and with labor and parts, went for 1000. It would have been 2400.00 at dealer, I happened to know someone who gave me a deal. Best bet would be take it to a reputable, honest mechanic in the area. Have them take a look. Sounds like the engine lost time...