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"Think" is an intermittent problem. Good time to get a compression gauge, inductive pickup timing light, and 12V test lamps (LED and incandescent). These give definite information and are as faithful as a dog. However, interpreting their results requires thinking.
Low compression in a cylinder can cause marginal ignition condition.
Check each plug lead and coil HT lead for regular pluses.
Check low side of coil for regular low voltage pulses to ground.
Instead of a new "good" FP relay remove the relay and jumper the 12V and pump socket pins. 1/4" tab crimp terminals fit the socket pins perfectly.
Since the tank pump has never been changed take advantage of the opportunity to help solve the problem. While the lid is off the tank look inside with an LED light to check for crud on the bottom. It should be shiny and clean. Don't switch anything electrical on or off near the open tank.
