Meanwhile, I have noticed that the relays' erratic switching can be coaxed into operation with a gentle flick (an old trick!!). Seems to suggest that the contacts in the relays have somehow worn out, even though they have only been driving the actuator for less than 1 month.
When the 12 VDC to a motor starts becoming intermittent you may want to look long and hard at all of your connections. A loose or faulty connection will cause the symptoms you describe, especially under a higher current draw. How much current does your actuator draw under load? It's just a little peculiar it works fine sometimes and other times your 12 VDC drops down to 7.0 volts. If a breadboard is involved in this with wires poked into it I would wait for it to give the low voltage symptom and start wiggling and jiggling your connections. Nothing to lose trying that. Ron
I have not seen any decoupling on your diagrams. The "gentle flick" provides a litle extra energy that allows an under-powered coil to actuate a relay. So I'd look for possible issues there, and definitely check for adequate decoupling, especially at the relay board.
OK, so we are clear on how the relays and the bridge are wired:Diagram with the assistance of www.circuit-diagram.org.
Please don't get confused by all that stuff about bridge rectifiers and protection diodes; I'd suggest all you really need is a suppressor cap across the motor terminals.
John ...... will a suppressor cap (with no flyback diodes being used) prevent the relatively large reverse-voltage upon switching off? And also prevent the sparking/arcing of contactors upon switching off?
@joatmon13, could you please post links to the specs of the devices you are using?
On Arduino relay boards I have found online the relays are labelled 5V so the activation energy comes from the 5V side. I'd check the 5V dc supply and make sure it can supply enough current to activate both relays at the same time.