Bad boot. Spray it with wd-40 ( displace and moisture) see if it goes away. Might me a carbon or contamination tract that holds moisture. Has it been raining where you are?
in any event, the boot now has poor dielectric properties (holding moisture or contaminated, or even punctured) and should be replaced,
69 X-44 base coupe, ZZ4, 16lb nodular flywheel, Center force DF clutch + PP, Holley 750V/S, Hooker super comp 1 3/4 ceramics, 2.5" dual exhaust w/ X-pipe, flowmaster 40 series mufflers, Aluminum radiator, spal 16" electric fan, Tremec TKO, 8.2 10 bolt 3:55 posi rear. front and rear Hotchkis suspension