PowerTap: is mine broken?

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I've got an old black-hub PowerTap (the CPU was black too until I got a newer yellow one) which I've had for about four years and bought second-hand. It's also seen regular use clamped in an indoor trainer this past year.

I was doing 2x30min threshold intervals today and for the first interval all was fine. My FTP is 253W, and my average for the first interval was 245W. I had a 10-minute noodle, then got ready for the second.

Immediately I realised something was wrong with either me or the PT. I was having trouble keeping above 200W even though my RPE felt about right. I thought I might be a little fatigued, but when I got out of the saddle to get over a rise, my numbers refused to go above 200, occasionally falling to 160W. On a 53x15 (don't be impressed, I had a tailwind) this seemed odd. I stopped, moved the sensor closer to the hub and continued.

This seemed to work for a while but again it started going nuts, jumping from 210W to 245 or 250W and then down again. I kept plugging away and managed to get an average of 226, but the numbers were jumping around while I was pushing along at a steady rate. I've been doing these intervals a while now so I have a good idea what they should feel like.

There were no power lines and I've used the same route trouble-free before. I had also zeroed the torque before the ride and did it as I coasted when I noticed the problem.

As I noodled home the wattage seemed OK. It's just when I put some power down it seemed to go haywire.

Would this indicate a torque tube problem? Have the hours on the turbo killed it, or perhaps old age has caught up?

I'm going to try new batteries, but I'm not optimistic this is the solution as the transmission icon was on all the time.

If anybody who has had torque tube issues could share their experiences I'd be grateful.

Alway suspect batteries, easiest to check anyway.
Are the cones loose/tight?