2005 code PO300

Hi,
New guy here. Been researching for a solution to the TDC PO300 that's being put out. It seems like I've read a million post,but haven't found one quite the same as my trouble.
Just bought this car about a month ago and can't get it to pass Pa. emissions. Car only has 13000 mile on it and runs really well. My problem is the 300 code and it not shifting into overdrive. The mil isn't lit and I thought it might be burned out, but it lights when the ignition is turned on. The hold light doesn't come on when the ignition is first turned on or when the hold button is pushed.
I've changed plugs to autolit and replaced the coil and plug wires, but that hasn't seemed to make any difference. I'm kind of stumped as to what to try next. Any help would be greatly appreciated.
I've read some great advice on this forum already and hope to be an active member.
Thanks!

Today I took the throttle body off and cleaned it. It was really dirty behind the throttle plate. I cleaned the vacuum ports and the sensor areas. Checked all the wires going to the injectors, none seemed loose. Checked all the vacuum lines and didn't find any cracks. Put it all back together and in less than a mile got the PO300 code again. Very frustrating.

-p0300 is multiple misfire?

you need to spray the coil pack and wires with a mist of water while it is running and look for the light show..

Thanks. I tried that. Nothing showing. Just replaced the coil and the wires, so hope their both good. I didn't get any arcing off the old set, but thought I'd try a new one anyway.

did you check the plugs? is it strickly a p0300 code? and can you feel a misfire?

try a compression or cylinder leak dow test?

The 300 is all I get. I took it to a shop and that was the only code they got too. I don't get a hold light when I turn on the ignition and nothing happens when I press the hold button. However it seems not to be shifting into 4th gear. I'll try the leak down test tomorrow and see how that comes out.