error code 221

Have a 1994 DC912 that has recently started to show error code 221. Machine runs for a few hundred or sometimes thouand stitches and then 221 lights up . I re- set the red mark with the arrow on the drive pulley (slightly off), restart and it goes agian for a bit and does the same thing? Longer the machine is on the problem seems to go away?
221 is the code for the X+ limit sensor but it doesn't seem to be the cause?
can anybody help me?

thanks
Britt

Location: 
United States