Instead of cleaning the old plugs, try installing a new set of platinum
plugs (maybe even the Bosch dual ground electrode platinums) along with new
wires. (In the old days, an oil coating problem on the plugs could be fixed
by installing plugs one heat range hotter than stock -- assuming you don't
have any overheating problems. Don't know if that can still be done with
modern plug heat ranges & all...)
Also, a vacuum leak you can't hear can still cause the mixture to go lean
enough to cause problems (especially as cars these days run at the ragged
edge of lean normally). If the vacuum hoses are old enough, they could have
cracks leaking enough air. Either spray them with something like WD-40 while
it's idling & see if the idle changes (a sign that they're leaking), or just
change them all on general principles. The kind of driving you describe can
produce high under-hood temps that will dry out the hoses faster, leading to
air leaks...
And then there's always the "re-set the computer" gambit. Lots of things
change as the vehicle ages and wears, so the computer needs a fresh look at
it all. Disconnect the ground cable from your battery and leave it for 10-15
minutes (or longer -- go for a snack). This should allow the pre-sets to
evaporate. Re-connect the battery and when you next start up the engine, it
will run oddly for a minute (more or less) as the computer gets fresh
readings from all the worn stuff and implements new settings based on these
now up-to-date readings.

Mitsubishi engine?  Not that great an engine.  Burning much oil?

On Nov 6, 2:54 pm, fleshbobroundpa...@yahoo.com wrote:

Since you have a 1990, do you know if the valve seals and guides have
been checked? 3.0's of this vintage had a problem of the seals
leaking, and the value guides sinking into the heads. These problems
were address later, before the 3.0 was discontinued. If you go to
allpar.com, there is extensive information about this

-KM.