When you go to a doctor, they usually take blood pressure readings. Its just a matter of standard practice. With any newly acquired Mac in the compact line I always re-solder the areas where connectors are. On the logic board there is one. This one is almost always bad with cracked solder joints. Then on the analog board you have three. The same one going to the logic and two for the CRT. These often have slight cracks. Slapping it on the side does not always work, in fact if it does it simply shows there is an issue that MUST be addressed. If you don't fix the solder joint they can cause issue, such as burning of connectors.

So before you go further, these must be checked to rule it out. Then one can move onto any other problem solving.

Just my 2 cents though. Years past I worked on many compact Mac. And recently I acquired and serviced several more. I would have to say all had cracked solder joints from one degree to another.