N.R.
R.,
Have you tried the Maclaren? I have only heard GREAT things about it!! And it holds up to 65lbs and is very light weight! My son is 7 mos old and I have the Rock Star Baby which I love but once we go in to an Umbrella stroller I willl be getting a Maclaren. My son is only 7 mos and the lenght of a 15-18 month old so I am assuming I will have the same issues as you!
Good Luck! Oh and I also think that RSB has umbrella's you may want to look them up too....