[niles]

Luc;

Your description is of a fairly common elect. problem.
First confirm your battery is staying at 12.8 volts; ie, check battery directly at the posts with a good digital VM cold(when it always starts) Should be between 12.5 and 12.8 V, then drive
until it gets hot and doesnt want to start. I'll bet you will still have 12.8 volts on the battery posts. If your voltmeter says 9 thats just a symptom of the problem.

When your engine gets hot it expands and moves and is making a bad connection on either a hot power terminal(solinoid or starter terminals or ) most likely a ground). When the problem prsents itself, quickly feel all the high power connections and grounds, the culprit will likely be hot. In any case remove, clean with sandpaper or wire bush all connections, including the RUST ON THE CHASSIS GROUND CONNECTION POINTS, coat with grease and retighten. Your problem will disappear. The only other possiblity is the starter contacts are being over heated from your headers, this can be remedied with a metal plate or insulation blanket over the starter
cheers
gn