96 Ram 1500 - 318 - Oil light

Ok guys, I have been reading your posts for quite some time now and I
have used alot of the good advice here. I am at my end with an oil
problem.

I have a '96 Ram 1500 Quad Cab 318 5.2L.  Last month I had the plenum
gasket replaced.  After that, my check engine light came on and the code
came up as the EVAP code.  They checked all the hoses on the intake
and cleared the code.  Then just two weeks ago I had the thermostat
changed
out because I was having erratic heat temperatures coming out the vents
at idle.

Now, I have my oil light coming on. This only happens if the truck runs
for a good 2hrs in city driving.  Above 40 at cold and after warming up.
After the 2hrs, I come to any stop sign or a complete stop and the truck
is at idle.  The light will come on and the needle will drop way low
(enough
for the lite to come on) and start to flicker.  I checked the oil level
and
it is at the "safe" level.  I know it blows blue smoke but it has been
doing
it for years.  Oil pressure gauge jumps back to normal while driving.

I am at a loss.  I had one place tell me it might be the pistons/rings or 
valves
that need replaced.  I am not sure if it might be a bad job when they 
replaced
the plenum gasket.  Just at a loss and would like to know if anyone else
had this problem or if anyone knows what the problem is.

Hate to get rid of this truck, since it has been a great truck and with 
almost
150,000 miles on it.

Any help would be appreciated.