It would be better to organize the interrupt routine to process when the clock edge occurs and poll in then just that bit of data. Have the interrupt manage a simple state variable that keeps track of such things as:
0=Waiting for start
1=waiting for 1st bit
...
...
...
n=Waiting for parity.
n+1=Last data received
The interrupt routune would then do nothing
but fetch data and setup for the next interrupt and state.

Timeouts can then be managed in the mainline code, where it may even be useful to use a timer interrupt and software timer counters to determine timeout period.

Michael Karas