Picolog thermocouple temp reading off

[irunbird]

I've recently had a major downward shift in my temp readout from this expensive gadget, and wondered if thermocouples need replacing after a few years of use.  This is a J-type TC that I bought and mounted in the drum of my roaster, and it worked spot-on for the first 2+ yrs of service, and it seems to be off by appx 15-20% now.  Another thing, the readout has about a 2-5 deg F fluctuation between readings that it never had before.  The old temp curves were very solid, but agreed with my contoller readings.  Has anyone had a similar experience?  I don't have another digital meter capable of reading J's (I do have one for the K-type, though...).

Ray

[blzrfn]

I've never spent more than a Jackson on a thermocouple, but I have had a handful of those that I replaced because they started giving me funny readings, either too high/low or jumping around a bit when they got towards the higher temperatures.  I have had both bare thermocouples and thermocouple probes that have done this.