For the fuel tank vent, as it starts to stumble, loosen the fuel cap. If it clears up, you have found your problem.
The coil is trickier. One way to check if your coil is breaking down as it heats up is to cool it down.
Head out to Staples or Office Depot and pick up a can of air, you know, the stuff they sell for blowing the crumbs out of keyboards and the cobwebs out of your cooling fans.
When the engine starts to falter, or shortly before you expect it to, hold the can upside down and spray the coil for a few seconds. The propellant in the can will be super cold, so keep your fingers out of the way.
If the engine runs normally, you can be pretty sure you're on the right track.
