After powering on monitor says no sync - Compaq Presario 5000 PC Desktop

I've almost the same problem. When the system turns on the monitor doesn't sync. After a variable trials with down/up the sync is enabled and the computer starts.A very clear event occurs : when the monitor can''t sync ( and system doesn't boot)you can hear very clear a high fan rotation. When I look/hear in the system the Isuspect the cpu fan. The system boots ( monitor syncs) when the fan rotationgoes from high to low. So the system hangs on an internal check on the rotation speed of the cpu fan.I've googled a lot , but until now doesn't find an answer . Possibilties which I think about : replace cpu fan , clean cpu fun ( but it looks clean).I cannot measure the fan rotation speed when the system is hanging. At normal rotation all info can retrieved witth the program cpu-z.Did you already have a solution ?