Are you working two consecutive decreases on either side? (Sounds like you are.)
Depending on whether the stitch before the decreases is a knit (or purl), work one purl (or knit) decrease followed by a knit (or purl) decrease. This should keep you in the seed stitch pattern.
Example: K P K P (K P) (K P) K P K P -- you have to decrease the two sts in parentheses: work one as a knit decrease (e.g., K2tog) and the second as a purl decrease (e.g., P2tog)
The next row should look like this: (the decreased sts in parentheses)
K P K P (K) (P) K P K P
~Dawn (but I go by WF on the 'tubes)