problems decoding MP3

I am fairly new to MP3 encoding/decoding but I have a task to add this
to my project.  I first implemented encoding and as far as I can tell
that is working as expected, but my attempts to decode have hit a problem
I do not understand.

Decoding a mono MP3 file is OK except that there is a short silence at
th start.  When I try to decode a (joint) stereo file the left channel
is correct (apart from a possible delay I have not chased yet) but the
right channel is not.  In the case of a simple sine wave I can see
apparent jumps in phase with a regular pattern of about every 1100
samples.

I am finding the documentation of the API a little difficult to
follow.  What I am doing is reading raw data into a buffer and calling
hip_decode1_headers which seems to read the header information as
expected.  I have tried looked at the left and right arrays without
enlightenment.

After the first frame's worth (?) I use hip_decode1 after reading more
raw data in a loop as I think the header should remain.

Just to be clear I do no want to decode the whole file at once but in
sections of my users' choice.

I would appreciate any suggestions, comments or corrections.

==John ffitch