When you have this problem it is always either the odd or even transponders going out at once. In other words you will only see evens all working or only odds working. Usually it will default to 18 volts and not switch back to 14 volts. In order of problems I have pesronally noticed.

1. Bad LNB
2. Bad cable or bad connector.
3. Bad multi-switch.
4. Bad IRD

Doubt it's an IRD because both of them do it. I have seen this before but it is rare. I would also think it is not an LNB because both IRD's are exhibiting this problem. But I have still seen this happen so don't rule them out. I would lean towards the multi-switch. I think you said you changed it but I would still look into this. I would look into a powered multi-switch as they seem to be better then the passive switches. If it is powered make sure you don't have it plugged in with fluorescent lights.
How long are your runs? Eagle Aspen makes a good switch as does Channel Master. Spawns are a lot more expensive and actually have the most problems in my experience. Make sure to check your cables and ends. It is always one of these things I have mentioned. A cable splitter will also cause this but you would have it all the time and not just intermittant. Sometimes we will switch IRD locations but since you said both are doing it I would think it is something common to both sytems. Elimnate the multiswitch and run the wires from the 101 LNB direct to the IRD's. You would lose the HD channels but at least you could narrow down the problem. Good luck but I am sure you will find it.