K.K.
My youngest daughter is a twin and was premie by 7 weeks...so she wasn't in the 90th percentile on height and weight (still isn't actually) she wasn't even on the chart...lol...but...she did have the same experience you are claiming when she was about that age....honestly...the ONLY place we could get her to sleep for any REAL length of time, including night was in her swing.
She needed the movement for some reason. I do not understand it, but that is what worked for us...AND she was not happy in the travel swing, it HAD to be the big expensive swing and set to side to side instead of front to back...best $70.00 expenditure we made...it saved us a lot of sleepless nights once we figured out that she would sleep there and accepted it...
Good Luck!!! ;-)