Then don't use a WAIT qualifier in your SERIN.

If the problem is a noisy receiver, then you will get data from the SERIN if you read every byte without Waiting, although most of it will be garbage.

Compare the "pp3" sequence manually, and once received, grab the next byte.
In the compare loop, do the timeout manually too.

Also, put a timeout in the SERIN statement, in case there isn't any noise at that time.

If the problem is that it locks up due to opposite IDLE state. That will not help at all.