I recently picked up a 1987 Phazer. Yesterday we had it out it once and a while the sled would lose power and throttle response. We checked the plugs and they were a nice cardboard'ish color. Sled would start and idle nicely just no throttle response until it seemed to cool down. Any possible ideas as to the issue? Thank you in advance. P.S. The outside temperature was around zero celcius.