It has happened 3 times now.  Once in July, once in Aug and again Sunday.  At Fl 240-250 when I pull on the cabin heat, occasionally  the manifold pressure starts to slowly fall off and the airspeed declines.  (30” down to 26”)  If the cabin heat is turned down or off, the manifold pressure returns.  I replaced the SKEET tubing from the hot side to the mixer valve as I found 2 very small leaks.  I thought this had fixed the problem.  The problem returned Sunday.  It does not happen all the time.  It does not happen at FL130-160.  Any clues as to the cause and where to look next?

Carl Cadwell

N25CL, IVP 870 hours on TCM TSIO 550E