That is common out of balance problem when it comes in and goes away between speeds
I would take it to an independent tire shop after a call describing your symtoms and have them check your wheels for problems.... I went thru same thing on a brand new SE-V6 Nissan and those jokers just gave me the shuck and jive bs too...so I paid for an independent to do those idiots job and found out I had a defective mag that was warped as well as a tire that had slipped belts and made them order me a new one and pay for my out of pocket expenses after mentioning the "L" word (lawyer lol)