I've never had a problem with lgrP degrading while being stored, but I've never stored any in an attic, either. It seems to me to be a very stable compound, and seems to take being left out to air dry in an open container in a hot room in the summer without a problem. I store what I have in plastic airtight containers. I have never found that it mattered whether it was stored dry, damp or wet, although I always rinse the red with hot water several times after using it and allow it to air dry before putting it in a tupperware container.
If it clumps and does not break apart easily I clean with alcohol followed by acetone rinses. If its dark after a reaction, I rinse with acetone until the rinse acetone is fairly clear. If the the red is really dirty, waxy, or tends to clump, I may boil it in HCl.
The only cautions I might give are to avoid extreme heat, store it in a sealed container, and store it well away from iodine.
If you have used the red in a reaction, your problem may be contamination from the last reaction rather than degradation of the red phosphorous. Try cleaning it well before concluding that it has degraded in storage.