Rich,
A possible explanation...
The tank fuel enters a common chamber within the petcock, and from here exits straight out the 'aft' nipple, and via a couple of 90 degs bends the forward nipple. With the petcock ON all is well...
When set to RES, fuel only flows out the Aft nipple (easiest route), but not out the forward. However, if the Aft is restricted, it'll then flow normally out the Fwd nipple...
So... this implies (to me) that there is a restriction in the reserve tank feed, ie it is unable to supply enough fuel to BOTH forward & aft nipples simultaneously.
Ie... The reserve filter is likely clogged, or it's fuel gallery clogged prior to the common fuel bowl.
Perhaps a new filter, sonic clean or air line?
Simon