Well there you go affirming some recent research, Novice2,

Traditionally, doctors concerned about blood flow during sexual intercourse have recommended a sex position with the woman on top. Actual studies that measured blood flow and heart rate during intercourse did not find this position to be any better than others.

In terms of propulsion, however, you will get much better thrust if you are actively using your pelvic muscles rather than just lying down. So keep thrusting away until further notice.

Nope, you're not alone on that one,
The Love Goddess

I have found that I have a hard time sustaining an erection while lying down, for example, when the provider is in CG or similar. However, in mish, doggie or other positions when I am standing or on top, I am good to go. I don't think this is psychological. I have no medical credentials, but could this have something to do with blood flow to my penis?  I.E. "downhill" versus "uphill" flow?  Seems like kind of a "limited circumstances" ED.  Havent, and don't really want to, resort to Viagra and the like. Was just curious if you or anyone else had ever heard of something like this. Thanks.

One corollary here:  your erection is ok when you're in the dominant position (you mention Mish & Doggie are good); problems occur when you're the passive partner (lying down for CG).  

You deny any psychological issues, but it sounds like you need to be in control to keep that erection.   I don't care for the passive positions either, it may just be a personal preference.  I wouldn't worry about it,  unless you really prefer your partner to take the lead.  --z

Easier to control the level of sensation that way when wearing a condom.