Hi John,
You might want to check with a pulmonary specialist rather than your rheumatologist for a second opinion on the declining pulmonary function tests. While there can be some variability, a decline over time in the test results needs a more comprehensive evaluation.
You might also want to check the comments on MCTD that accompany my articles on MCTD and undifferentiated connective tissue disorders on suite101. Be sure your doctor has run a vitamin D level too for 25 OH vitamin D and is providing treatment if a deficiency exists. Best, Elaine