I've been chasing this same ghost for a couple of weeks. Sucked up some sand, clogged system, then overheated (slightly). So while flushing the P/S and oil coolers, I noticed that the starboard riser was much hotter than port - by 40 ~ 50 degrees average. So I reflushed and reflushed and ... scratched my head and cussed and....

I don't believe in ghosts so I've tried to forget about it. Haven't melted anything yet.

Port - about 105 degrees running
Star - about 150 degrees running

I'm putting the new housing on next week and will see what happens. The old one seems to have a good amount of rust and scaling, possibly some partially blocked passages.

After that - I give up. After all, even the hot riser is running around 138 degrees, it's not near cracking temperature. I've read on several sites that anything under 160 or so is fine.

Update:
Decided to use clear hoses from tstat housing to each manifold. Noticed minimal bubbles on the side that was running cooler, and much more on the side that was running hotter. Took the hose that should feed the port manifold and moved it to starboard and vice versa, and the problem flipped to the other side.

So, do you think there's a chance that I have a small crack in my tstat housing that is introducing air as the water exits the housing? I would think that if the air was being introduced anywhere prior to the tstat housing, then the air bubbles should be coming out evenly from the housing to both manifolds, no? Mine is not even at all, one side is getting much more air bubbles and that side riser is running hotter and blowing steam.

I too have had this issue for basically forever on my 502, changed parts, did all sorts of things and never found the "issue" as others have said I think it's normal (or maybe that a nice way of saying that I gave up :)). I see about a 25-30 degree temp difference and the starboard side of my engine kick out tons of steam compared to the port, best friends formulae 232 (stock 454/330) is exactly the same as mine. As long as you have good water flow, I wouldn't worry about it to much. Though someone here may say different and have a great idea that I wan't able to sort out. I too was able to make it swap sides FWIW.

As far as the bubble issue, that may be something entirely different.

here is a possible cure. get a 4-way adaptor fitting. run both inlets in and both outlets out. then the water will mix and even it out. if you don't like the way it looks, don't open the hatch...

my best guess is casting inside directing flow to one side over the other, i have noticed this in older tee with control balls but not as much temp difference. the rear water passage trick is same as done on 6.2/6.5 diesels. tap holes into rear of intake; most intakes have dimples already. and put fitting and hose, either to overboard discharge or to t-stat spacer that cpperformance sells. allows air to vent rather than travel thru heads.