Tsk, tsk, tsk. You sure you want to keep at this???

Check all electrical connections for tight and corrosion, both at starter and batt. Especially if your batt is in the trunk (meaning long cables and resultant load), given boy racer history of your car it wouldn't surprise me. Also, if it is remote you could have marginal gauge of wire too.

Just because the voltmeter shows 12v at no load doesn't mean it's fully charged. The drop to 0 shows what volts you're getting under load. Again, it could be high resistance from bad connection or "light" gauge wire. Also, with all the start cycles you did you may have drawn down the battery more than a 45 minute run cycle could compensate for, especially if your alt belt a a bit loose, or if there's corrosion at the terminals.

That should keep you busy for a little while longer , then let us know what you learn..............................again.