I had a post a while back about this but I got a better explination now and tried some suggestions, but to no avail. 1981 w/'78 350/TH350c/NP208. On every third gear shift the truck feels like the RPMs are cut in half and the exhaust tone gets lower. It loses all guts too. When it does this and say I'm pulling an incline, it'll starting shaking and shuddering hard. Once its up to highway speed, its smooth sailing. I have replaced the vacuum modulator, vacuum advance, and fuel filter. I'm really running out of patience on this and its driving me nuts. It was only intermittent at first but now it happens on every third gear upshift.