If it's depending upon GPS for initial elevation, than 100' isn't too bad. It seems that GPS elevation is derived differently than location, which could contribute to the variety of results. Without any calibration, you could see anywhere from 500-1000' differences in elevation over the course of just a day or two which is perfectly normally.

However, altitude drift like the picture below on a day with no pressure change can drive you nuts (this was an out and back route which took place in less than an hour)