I had a similar problem! I have MSD HEI upgrade kit, brand new plugs and wires, etc. I started the 85 C10 up (it has a 305 too) and moved it in the driveway. It ran fine. I cut it out, pulled the 66 out and then went to put the 85 back about 10 minutes later. I start it and sounds like water in the cap and not firing right or cylinders were out. Well I didn't check the plugs but a guy told me to. Instead I just started it, it idled like it was way off timing and didn't idle too long. I started it again and help the gas and let it sit reving for about 3 minutes to clear it out. Left to help a guy build a 302 and probably 5 hours later came home and it started fine. No problems anymore. The guy I bought it from says it fouls plugs....