gaps between rows--I switched f garter to stockinette on cir
 

I started knitting a sweater from the bottom using all knit stiches on a circular needle. When I bound off the armholes, I switched to stockinette between the armholes (still on circulars) and now I have a weird pattern developing every third row. There is a long gap between rows every third row- it is very predictable - like a pattern, but I want evenness in my stockinette!

I recently switched from a combined style of knitting to continental knitting- but I didn't change the way I was purling.

Or could it be weird tension from the circulars?

Any help you can give me is really appreciated. Thank you!
Here is what my sweater looks like:

top- (working edge)
VVVVVVVVVVVVV
VVVVVVVVVVVVV
VVVVVVVVVVVVV

VVVVVVVVVVVVV
VVVVVVVVVVVVV
VVVVVVVVVVVVV

VVVVVVVVVVVVV
VVVVVVVVVVVVV
VVVVVVVVVVVVV (armhole bindoff here- switch to stockinette)

VVVVVVVVVVVVV (body - all knits on circulars)
VVVVVVVVVVVVv
VVVVVVVVVVVVV
VVVVVVVVVVVVV
VVVVVVVVVVVVV
VVVVVVVVVVVVV
VVVVVVVVVVVVV
VVVVVVVVVVVVV
VVVVVVVVVVVVV

I've heard of gauge changing when you go from knitting in the round to knitting straight, but every third row? That is odd. I can't think of anything to help, just to agree that it's odd.

You may have "Rowing Out". I struggle with the same thing. One tip I read was to switch to a smaller needle when purling...easy to do on interchangelables...not so much on regular circular.