HLS30RACER

The clunking makes me think the problem is likely the mount for the front of the differential. when the rubber sandwiched between the metal plates finally gives up it exhibits symptoms just like you are describing. You said everything visually looks good in the rear, did you try prying on/ jacking up the nose of the diff? it often separates where the rubber is glued to the metal and therefore looks fine without further probing. it takes considerable force to move it up and down even fully separated.

I experienced the same symptoms a while back, it turned out to be the ballast resistor (mine was mounted on the coil itself). it is much easier to test the resistor so I would start there.

I had exactly the same problem a few years ago only mine quit running while doing 65 down the freeway. It turned out that my ballast resistor on the coil had gone bad. It would start and run with the key in the start position because the ignition switch on the original equipment bypasses the voltage drop to the distributor to increase spark power during start-up. Worth a check...

First convert the displacement of the engine to cubic feet by dividing the number of cubic inches by1728 (the number of cubic inches in a cubic foot). Ideal flow will be the cu.ft. displacement multiplied by the engine RPM divided by 2 (divided because its a four stroke engine). Now factor in volumetric efficiency to determine actual flow in cubic feet per minute. Displacement(cu.in.) x Engine RPM/2 x Vol. Eff. = CFM 1728 cu in./cu ft. Hope that clarifies the issue a little.