'92 300SE

Starts immediately when cold. When engine is at or near normal operating temp, starts "indecisively". I turn the key, motor cranks freely but doesn't "catch" like it does when it's cold. Sometimes it will catch and run ok, other times it catches and dies immediately. If I give it a bit of pedal when cranking, it fires right up.

This is a new behavior - is there something wrong or something I should be checking?