Almost ready to pull trigger

[dboyd351]

The cooling passages in the risers can become clogged with rust as they get some age on them. Pulling them and cleaning the rust out may well solve the discrepancy as long as 1) the bolts aren't so rusted to the manifold that you can't get them off and 2) There isn't so much corrosion the risers need replacing.

That might not be what it is, but it sounds like a possible reason for what you are describing.

Having a mechanic check it out, as previously suggested, is certainly a wise decision, too.