Should a second layer of asphalt be placed over an older layer of asphalt?
Another layer of blacktop asphalt, usually a finer "grit", will fill the low spots and get you through another year -- but it sounds like your driveway is under-engineered for the weight you're regularly driving/parking on it. So, you've got a few choices: 


Another layer of blacktop every year or two, and probably mopping tar on it every summer. 
Tear it up and put in a new asphalt driveway, which will probably have the same problems. I would make sure you consult an engineer and have a few soil cores/studies done before you pursue this angle... you will want to have the substrate rebuilt from the soil on up, and you will want to have had some soil studies for expansion ratios and whatnot done and an appropriate base engineered before you pursue this angle. This will PROBABLY include digging up everything that is there -- the blacktop, crushed gravel, and all, re-tilling the soil with some additives to improve it's compressibility for your climate, and then re-laying everything... probably a bit thicker. 
Tear it up and put in a concrete driveway with a high PSI load concrete, which will occasionally have problems due to the freeze/thaw cycle, but won't sag due to vehicle weight and where you can have any lippage that appears ground down and then can cover it with blacktop again when it does finally blow out or get too thin to grind. See notes above about soil+engineering study, plus tearing out all the base layers and digging deeper/re-packing them. 


The last is the pattern that will last the longest, but is obviously going to cost most.