97 Ford 4.2L V-6, 5 spd stick P0306

I'm getting the check engine light (P0306, misfire on #6 cyl) every time I'm running in the 1500- 2000 RPM range with very little load on engine. The engine doesn't miss, still gets 18 to 20 mpg........... runs perfect for 200,000 miles. I'm thinking that #6 cyl. is a little weak. It also uses about a pint of anti-freeze every 3months which has been going on for 10 years.

FIXED.....took out the Champion spark plugs (heat range 14), replaced with autolite APP105 (heat range 5)