Re: How does a liquid's viscosity affect it's boiling point?

First, I can say that your hypothesis is *mostly* correct.  As you have 
confirmed, a higher viscosity *usually* means a higher boiling 
temperature.  But not always; for example if you tried the hydrocarbon 
octane (which you shouldn't because it is flammable!) you would find that 
it was less viscous than water but had a little higher boiling point.

In science we refer to this as a "correlation", when there is a tendency 
for two characteristics to appear together but it isn't an absolute rule.  
A simple example would be that there is a correlation between being tall 
and being a professional basketball player.  There are a few shorter 
people in the NBA, but on average they are pretty tall.

If you call the two correlated characteristics A and B, the two simplest 
ways you can get a correlation are:
1) B is (at least partially) a result of A (or A is a result of B).  This 
is the case in the basketball example, since being tall is directly 
connected with being able to be a better player.
2) A and B are both results of some third characteristic C.  For example, 
there would be a correlation between driving a Lexus (A) and owning an 
expensive house (B).  There isn't a direct connection between A and B, but 
they are both usually a result of being wealthy (C).

The case you are talking about is more like the second category.  
Viscosity and boiling don't directly affect each other.  But they are both 
strongly affected by the forces between the molecules.  The stronger the 
attractive forces between the molecules in the liquid, the harder it is to 
get the molecules to "let go" of each other and go into the gas phase 
(boiling).  Also, the harder it is for the molecules to slide past each 
other (viscosity).  The correlation is not perfect, because the way the 
forces affect the boiling point isn't quite the same as the way they 
affect the viscosity (in particular, the physical size of the molecules 
affects the viscosity strongly, though the correlation still works pretty 
well because bigger molecules tend to have more forces between them).  So, 
to sum up, of your two effects, one doesn't directly affect the other, but 
they correlate because they are both affected by molecular sizes and 
forces between molecules.