Omaha, NE
The filter cap on that voltage is quite small.
If you have close to a zero load on the coil voltage then the cap is big enough to hold it high consistently. It is a DC voltage under light loads.
If you have a heavy load (e.g. any coil) then the cap is not big enough to hold it high and you will have a rectified AC voltage as presented by the output of the full wave bridge rectifier. With a full wave bridge rectifier - you should be seeing 120 pulses per second (assuming your mains power is 60Hz). This is what you see.
