Hi Soab. Sometime ago, I was observing a similar prob on the 757 as well. AP & VNAV were engaged and we were climbing thru FL230 when the FMS acted as if it was not bothered about IAS. Pitch was remaining constant and on the ASI speed was dropping. The Capt. switched to FL CH mode and then things became OK. After a while VNAV was engaged again and then things were normal. Crew were quite unsure of what caused this.
I personally feel that the 57 and 67's VNAVs are a little sloppy at times...