Starting problems w. 98 Lumina

Forgive this humble newbie for his relative ignorance...

I have a 98 Lumina and lately it has been having problems starting. The first time it happened was after a cold snap, and I assumed it was just a dead battery. I tried starting it, it wouldn't turn over, and just made that clik-clik-clik noise. I had it jumped, let it run a bit, then drove it home...only to have it happen again the next morning. I replaced the battery, and it ran fine for a few weeks, then it happened again.

This time it started oddly (slow and "grumbling") and the was dead the next morning. I had it jumped, took it to the dealer (still under the service plan). They checked for a parasitic battery drain or some other electrical problem but found nothing.

It was fine for a week, but this morning it took its time turning over, and I suspect I'll find it dead again, based on my experiences so far.

I notice it tends to happen on nights when I don't take the car out except for the commute home. On nights when I take the car out to run errands, it's fine the next morning.

Any ideas on what might be going on? It's a little past due for a tune up (around 1000 miles past) but otherwise kept up perfectly.

Thanks in advance....