There's nothing wrong with gripshifts, Imanda
I have gripshifts and they perform very well. (I do have rather expensive ones but most should be fine)
The most common problems with bad shifting are:
1) worn cables or cablehousings
2) incorrect adjustment
I advice that, before you actually go out and buy new shifters or a new derailleur, you check your cables, housings and adjustment.
If you want to upgrade your gripsshifts to new ones, I highly recommend the SRAM Attack's ... I have 'em and they shift like ... euhm .... butter?