A bike shop put a PB 8.0 computer on my new trike. Unfortunately, once I get over 18mph, the speed readings go nuts and oscillate from single digits to fast. The sensor is mounted upright (mostly) and I have tried to get the magnet to cross just one of the sensor spots on the sensor arm. I've adjusted distance to the magnet, and so on.

Anyone else overcome a problem like this?

This sounds like a connection problem. Slide the computer off and on it's mount a few times. This should clean the contacts allowing a good connection.

I don't think it's a connection issue. The contacts are clean and it happens with the trike on a table while I crank it by hand.

I don't think it's a connection issue. The contacts are clean and it happens with the trike on a table while I crank it by hand.
take it back and get a new one.

take it back and get a new one.

Very close to doing so, but it will have to coincide with another trip. The gas is a good percentage of the price of a new computer.:notamused:

I talked to Planet Bike. (Great customer service by phone.) They said the sensor should work fine laying parallel to the ground. That fixed it. With the sensor upright, I just couldn't keep both points from firing. The LBS was sure it wouldn't work on its side.