Yet another repair?When is enough -enough?

I have a 1998 ChevyLumina Van. It has 120,000 miles.
I just had the trans mission replaced ($1,200) I got it home and it was running hot. Took it back to AAmco. The said a head gasket was cracked. Replaced that ($3,500). It ran fine for about a week. Then it began having trouble again. It would start fine, then just sort of die quietly. It does this at stop sign and lights. It starts back up ok and once the engine is warm, the van runs well. I sort of lunges forwrd when I step on the gas, but not all the time.
Back to the shop. Now the guys think it is a 'cam' that needs replacing.
I'm at my wits end. The only reason I had the car repaired was it was my moms car. (She's dead by the way). I just can't keep spending money on this car. It's the most comforable car I've ever driven and it's my main transportation.
Should I spend any more money on this car?
Any ideas on what a cam replacment should cost?
Thanks for your time.