When your car is started and has had time to warm up the engine starts to rev high and then drop suddenly and just keeps on doing this for a while and then will stop and then start again?

Sounds like you have a intermittent with the Idle Air Control valve. Wires that go to the valve may be loose at the connector or intermittently shorting out to one another or ground. valve may be sticking, opening and closing from carbon build up. these valves can be taken off and manually cleaned.