David:
First you need to work out separating the Receive Interrupt activity from the actual process of interpreting and saving the ascii stream into the external RAM.

A common approach is to organize the receive interrupt so it simply saves the incoming bytes into a small buffer. This buffer is usually configured as circular queue (a.k.a ring buffer). The RX interrupt puts data in.

Then the mainline program will poll for presence of data to take out of the buffer and search for the acceptable command types and accumulate the data for the command data packet. once a whole command data packet is accumulated it can be stored into the external RAM.

It may be that if you are logging many sets of the data into the external RAM you may want to consider converting the count values from the command data packet into binary format so that you can pack the data much more densely into the external RAM.

Good Luck
Mike Karas