Supercuda beat me to it and said exactly what I was going to describe. you are suffering from classic "chattering contact" syndrome, in which case the circuit rapidly opens/closes as you hover around the switch's pressure rating. Two switches, wired and plumbed as described will give you a usable range to work with. This is why your home air compressor shuts off at 120psi and kicks on again at 90. If it didn't that thing would running darn near all the time. If you select your second switch to be 10psi below the rating of your current switch, you will have to drop about 10mph before the OD disengages. I think you may find that 5mph difference is a little tight where a 10mph may be more reasonable, especially with a CTD.