Anna, Lantus is not necessarily going to keep your BS steady – a lot of people do find that Lantus has some really bad peaks & valleys. You said you could see having a rollercoaster if you had a pump but actually, a pump can give you a steadier basal rate because you can program a bunch of different rates throughout the day and meet your bodies requirements.
Have you ever done basal testing? Basal is not just for pumpers – I did it frequently when I was on MDI. That is one way to find out how steady your Lantus is. Gary Scheiner recommends that your BS not go up or down by more than 30 points for the basal test. John Walsh recommends that it not go up more than 15 points but it should not drop by more than 30. If you go by Gary Scheiner, that is a 60 point spread and according to John Walsh, it would be a 45 point spread. Both of those spreads have some fluctuation in them and your basals would be considered correct if you met either of those.
Like Onesaint said, you also need to look at what time you ate dinner & even what you ate. Protein & fat digest slowly. If you ate a later dinner of protein & fat, that could be part of the problem. If you lay down after eating, that can also slow your digestion down.