Any SMS text over 160 characters has to be split up as that is the max. character limit for the SMS system (many networks also convert anything beyond 480 characters into an MMS message).
I know the problem of out of order message-portions can occur when the sending handset's clock is not in sync with the network they are sending over. One solution for the iPhone has been to make sure, under date and time in settings, to have "Set Automatically" ON (syncronized your phone with the cellular towers you are connected to). However, if the sending phone is out of wack, that may not fix it on the receiving end, as they messages get messed up on sending, then received in the proper sent order (which of course was messed up). Things often seem to get worse when sending across networks (eg. verizon to at&t).
You can find lots of internet posts about the phenomenon gong back years - no sure fire fix, or really any fix at all. SMS was never intended to work this way - it is inherently limted to single messages of 140bytes (160 seven bit characters), so it has to chop up longer messages. If the sending or receiving handset have messed up clocks, then the messaging apps can mess up the order, or if the tower at the time of sending has a clock out of sync, it can mess up the order, or if the transmitting network does not use a buffering system (or has a faulty one) to keep the messages in order, the order gets messed up.
Its why I use email over SMS just about 100% of the time - SMS is old, and inherently limited technology. The very fact that it even allows messages over 160 characters is a sort of software kludge, not a real fix.