Every Anxious Wave by Mo Daviau

Two second tier rock stars operate a make shift time travel business that lets you go back in time and witness rock concerts you missed the first time around. The problems start when one of them goes back to what he thinks is 1980 to prevent John Lennon's assassination but he ends up in 980 Manhattan. To get his partner back will take more knowledge than Karl possesses so he goes in search of a physicist from Northwestern who has issues of her own. Some interesting wormholes appear out of nowhere and emotions complicate things but basically it is a look at love, what if and time travel. In trying to help you have to make the decision They all forget the basic rules of time travel - don't talk, don't touch and don't bring anything back.