The sl1, k2tg, psso is a double decrease which takes 2 sts away every RS row. That dec is in the middle of the row, so R1 should be k42, dec, k42. R3 is k41, dec, k41. You knit 1 less st before and after the dec every RS row.
If the stitch number is not changing, then you must be adding a stitch every row. Look at the sticky thread here http://www.knittinghelp.com/forum/sh...ad.php?t=32886
and make sure you're not knitting 2 sts into the first st of each row.