Sluggish acceleration / Misfiring after 5 minutes traffic jam

Just installed a coolant reservoir tank to my champion 3 row radiator. It´s working better now. Anyway, the car is still presenting some sluggish acceleration when stuck on a 5 to 10 mins short traffic. As advised previously my car engine is modified to 2.6 with oversized pistons and i was wondering if using a hotter plug such as NGK BP5ES will solve the problem when coming out of idle in order to obtain a smooth acceleration or this has nothing to do at all?