flybymike- Assuming you continued to use your old encoder, that may be the issue. The very old units take 10 - 15 min to literally warm up and as they age it gets more like 13 - 17min. Before this the encoder could be sending garbage and upsetting the trig. Does the altitude appear on the trig screen as normal?
150 Driver
That Narco had a mandatory fix which can cause it to fail to work with some of the new radar systems - what transponder are you planing on fitting to replace it? I would strongly recommend one that supports Extended Squitter out of the box.
Rod1