30 years?!?!? You should have come here sooner. We've been online 20 years.
I would first determine if in fact (we should always work with facts) it is running on only 5 cylinders at start up. It could just be running roughly on all 6.
Pull one spark plug wire, start it. Put that back and pull the next and start it. Proceed (even it it has to sit overnight) through all 6 to determine which cylinder is faulty, if any. Of course when the faulty cylinder has its wire pulled, start up will sound "normal".
Then, work to determine if the fault on that cylinder is fuel, air, spark, or compression.
Let us know what you find!