As long as your COM channel never has errors you should be good to go without start or stop data.
How do you find the start of the first message?
OK back to the problem first,
1. What on the PC is sending the data,
2. What is the protocol? each packet consists of a <header><data><tail> your telling us about the data but we (and you) need to know the rest. And you are not using '\n'!.
After that we need to know what tpe of Arduino you have and how you intend to debug this. If your using the com port for data you can't use it for debuging! Does your Arduino have more than one com port?