Bayer Advanced 704050 Season Long Weed Control for Lawn Concentrate, 24-Ounce (Not Sold in NY) https://www.amazon.com/dp/B00KHVR3SY/ref=cm_sw_r_cp_apa_TqDSyb5WRNJ9D
Read the label. I believe isoxaben controls dandelions.
Round up makes a bunch of different products. If you're killing weeds and grass not in your lawn, the one I linked to will kill everything, and better than Spectracide.
What trouble are you having - do they wilt a bit and come back, or do you have new weeds constantly coming up? If it's new weeds, I'd give Preen weed preventer ( http://www.preen.com/products/preen-garden-weed-preventer ) or Bayer Season-Long weed control ( http://smile.amazon.com/gp/product/B00KHVR3SY/ref=pd_lpo_sbs_dp_ss_1?pf_rd_p=1944687742&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B0044UZB7Y&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1GDC9D9B2NNT3FY9312R ) a try. You don't apply to weeds, but rather in places where you don't want weeds. It will kill them as they germinate. Most of the time they'll die before they get big enough to even see.