on version 1.8.3, if I try to add an interval program, the sprinkler basically locks up.
This is strange. Would doing an EEPROM / hardware reset help? (although when you upgrade 1.8.2 to 1.8.3 it should trigger an automatic EEPROM reset). How did you upgrade the firmware: using the firmware updater tool that we provide, or uploaded the program in Arduino IDE?
There is no difference between 1.8.2 and 1.8.3 regarding the rain sensor functionality. If it doesn’t work in 1.8.2, I think it may be a hardware issue.