Hot Water Heater Problems

I have a Rheem Power Vent hot water heater. Model 21VP50E-1 A. The hot
water heater makes hot water and keeps up with demand. Problem or no
problem: main burner cycles on and off in about 5 second intervals.
Downloaded technical service bulletin 1602 for troubleshoot from Rheem.
Vacuum safety switch checked sat, 24V between 24V (GND) and terminal #5.
Also jumpered vacuum hose with need and same results, no vacuum line
blockage. Blower motor runs normal. Ignition control module checked SAT,
24V between 24V terminal and 24V (GND) terminal. Thermostat and
transformer SAT. Do I have a problem or is this normal. Thanks in
advance for any info/insight.

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