hey sorry about long delay! but YES it ONLY happens when the truck is MOVING. And i've also now noticed that it happens when i'm coasting downhill but only when i'm at close to 40 MPH. Coasting at say 20 its ok. wonder what the heck i did to this thing!

1952 Chevy 3100
235 (58-62) 848 head ('56)
Putt-putt-puttin' along