Engine has a lack of power can I fix it?

My truck listed above has a power loss problem. It is always hard to start cold or warm. Here is the good one, it only acts up during the day, never at night. The truck never runs hot, always where it should be.

Put on a new muffler now truck lacks power, like as if the muffler was pluged, could it be a bad cat or a deffective muffler. When drivving if you try to floor it it's bogging down as if it wants to die out let off and lightly press the gas pedal it drives ok. What can I do?

Thanks will try that in the morning will post back if any more information is needed. Thanks, Gary