Excessive Vapor/Moisture In DV Gas Stove Pipe

I recently installed a Enviro Ascot direct vent gas gas stove in my basement. Everything is hunky dory, except for one issue: water condensation leaking from the exhaust pipe.
I've used ICC ExcelDirect 5" piping, 4.5' up, then a 90 degree out through the wall.
What is happening is, when the stove is off, (with the pilot either on or off), then turned back on, water will run out of the inside of the 90 elbow once the stove is relighted.
Obviously I have an air leak somewhere that is causing warm moist air to condensate and freeze on the inside of the pipe(s). When the piping warms, the ice melts and runs out the joint.
The trouble is, I can't figure out where it's leaking. Initially I thought it was being caused by the lack of good seal on the exterior of the pipe at the stove. (It being the lowest, and best point for convection to take place from.) I slathered on the RTV on that joint, but it did nothing.
We've had some rather frigid nights here in southern Ontario (-28C ) the last little while, so the condensation has been rather voluminous.
Has any one experienced this, and do you have any suggestions as to how to remedy this? I'm to the point where I want to disassemble the piping and coat all the joints, inside and out with RTV....

I'm not quite sure what you mean. I have a proper termination cap on the exterior. The only way I could see the termination cap causing issue this would be if some how warm, moist exhaust was being pulled back into the pipe. The design of the cap is such that it seems unlikely this would happen.
Also, since the frost/moisture build up is happening whether the pilot is lit or not leads me to believe this is not the case.

No, the ice is forming on the outer side of the intake shield, on the exhaust side. I'll knock them off, but I doubt that's where the problem is. It was happening in early December, before we had any snow, and before the cold snap that has caused this ice build up.

The cap is 18" above ground. I'm keeping an eye on the snow build up. We had a storm with high winds out of an unusual direction that caused this uncharacteristic build up. I may have to install a window well underneath the cap just to be safe, but I'll do that in warmer weather

I'm using a 5" vent, as per my local Enviro dealer's specs.

I still think it's the friction fit of the external pipe that causing it. It's not airtight, and warm air is leaking in and convecting upwards and condensing on the cold parts of the intake. Either that or the seal on the exhaust somewhere near the cap is bad, and is leaking moist air into the intake. Although, since the condensation is happening whether the pilot is on or not, leads me to believe it's not the latter.

This is a bit more than a drip. It's more like 10-20cc once the stove heats up. I'm also of the 'no moisture is good moisture' mindset. I've done enough reno's to know what damage small drips over long periods of time can do to buildings. Right now I just have a wad of paper towels sitting on the floor to intercept the runoff when the stove is fired. I'm also concerned about the possible corrosion of the pipe. If it was all stainless, I'd be less so. I'm not even sure if it's galvanized on the inside.