your first post says it's a 98, yet your profile says 99...?
have you checked your valve lash? hmmm...I'm not sure if you can on a 98, but 99 was sohc, so you could on that. and since the correctly gapped plugs may not have solved it, Matt may be right that it could be a coil pack on its way out.
If it didn't do anything bad until temp was up, it may be time to change temp sensor.