Hi Steve,

There's just been a thread on this on the Elecraft-K3 group on groups.io.  First recommendation was to check the connection from your power supply including power pole connection. Should have direct connection to the power supply.

More likely is a problem with the connection path from KPA3 to KPAIO3 to RF Board.  Depending on the S/N, there might be tin connections that corrode.  The fix is to replace with gold pin connections.  I had this problem on my older K3 (S/N 1428) -- symptom was I couldn't raise power level above 12W.  Replacing pins wasn't hard.  I think Elecraft changed out these connectors after S/Ns in the mid 3000's.

Good luck,

Gerry, W1GD