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Electrical sometimes flickers while driving

A few weeks ago my '99 Saturn SL (120k miles) started rarely losing electrical in little bursts. Gauges zero, car beeps, but everything is normal. This would happen maybe once every few days during my ~30 minute commute. Last night I was picking up dinner and when I returned to the car it had no juice and wouldn’t start (not even the dome light would come on). So I found somebody to give me a jump and when I returned the car started up without jumping and without me having done anything in the interim. So today I check the battery (which is maybe ~3 years old) terminals and everything is tight and corrosive free. What can I do? It isn’t in the best of shape otherwise and so I don’t feel up to potentially dumping a bunch of money into some mechanic who may or may not fix anything.

I would remove and clean the battery positive and ground connections, both ends, also if you have any other lugs, like for jump starting, clean that as well. I’d also reseat the large plug (s) going to the ECM if it is under the hood. It’s obviously a poor connection somewhere, the trick is to find it.

…or the alternator is failing.
After 14 years of use, it is certainly possible that the alternator is dying.

How would a bad alternator affect the car having no power when engine wasn’t running?

A failing alternator is unable to properly recharge the battery while the engine is running, and as a result, the battery will keep going “dead”. After a few of these episodes, even a fairly new battery will be ruined, so I would suggest that you have the alternator’s output checked a.s.a.p.

Most big box auto repair/parts places will do a charging/battery system test for free. That’s probably the place to start. BTW, just because there appears to be no corrosion on the battery connection, there still could be. Sometimes an oxidation coat will build up which is difficult to see. It’s best to thoroughly clean both battery terminals and the insides of the connectors that go on the terminals with one of those inexpensive battery terminal cleaning gadgets. Like this: