Overpressurizing Coolant System?

First off: there is a (known to me) minor leak in the top of the radiator. A few
faint wisps of white smoke coming off the engine after running is normal for
me by this point, but the coolant loss isn't extreme and I just haven't had
time to replace the radiator yet.

Here's what's happening besides:
The leak in the top of the radiator is something like a blown seam. I have:
Flushed the radiator, burped the system, checked upper and lower
hoses(good), replaced the overflow reservoir and the hose leading from
it(because, perhaps unrelatedly, they were torn up pretty badly. What you
get for buying a used car).

I do not have coolant in my oil. I do not have water in my tailpipe. I do not
have white smoke coming from anywhere(except slight wisps from under
the hood, due to the leak in the top of the radiator, as stated above). I do
not have the smell of burning coolant(but have the occasional smell of
burning oil). I do not have power loss. My heat works like a champ. My
temp gauge never rises above the 1/4 mark.

The further or faster I drive the car, the more rapidly my coolant depletes
through the 'seam' in the top of the radiator. Sealing it shut resulted in the
seal being blown through.

Several months ago(during the summer) I drove the car cross country, and
was having overheating issues with it. I'd drive a few hours, and hit the 3/4
mark on the temp gauge and then park for a few hours to let it cool down.
Chalked it up to summer heat and, as it didn't seem to be getting any
worse, forgot about it. Car ran fine as an around-town delivery car after
that.

My best guess, is that the coolant system isn't flowing properly, resulting in
a buildup of pressure. That being the case, I don't want to drop a new
radiator in until I find, and repair, the issue causing over-pressurization. I'm
fairly certain the Temp Sending Unit is shot, as I should definitely see a
reading above 1/4 at some point. So assume that my temp gauge is full of
crap. What I'd like to know is: where do I need to go from here? I'm thinking
water pump and/or thermostat is faulty and in need of replacement, but I'd
like some advice from somebody a bit more knowledgeable.

Thanks in advance!