Spark power test is performed by two nails separated by 8mm gap. One nail is connected to HT coil wire, another one is grounded to engine. Under all circumstances, there should be spark. If no, maybe HT coils are overheat and lose their parameters. If they run ok when cold, try to test spark power when engine refuses to rev over 2k RPM. Btw how does it idle then? Is idle still OK or engine tends to stop?