i change my spark plugs to ngk irdiums today, it started to drive fine but after 15-20 min my car starts bogging in 3 and 4th gear. Any idea whats going on ?

 

Are the plug wires secure?

 

yea its secure tight

 

check gap, are you using oem wires?? are you using the right plugs and make sure they're not backing out. pull spark plugs and check for correct gap..

1. i use anti sieze on threads
2. i turn it with torque wrench till 13-13.5 ft/lb or whatever the specs i think i got it right
3. i add a bit of dialectric grease on all connections

your wires might be too old, check cap/rotor as well