Check the throttle vacuum valve. The purpose of this valve is to trick the TH400 into thinking it is behind a gasoline engine. At idle, you need 17" of mercury vacuum, dropping off to 0" at wide open throttle. A cheap vacuum gauge and a few assorted vacuum tees will go a long way to diagnosing your problems.
The throttle position valve will leak vacuum so it has a little polyurethane filter to protect the vacuum system. This filter loads with dirt, eventually decomposing and causing the valve to fail. Simply teeing into the TOP port with the vacuum gauge and then going on a test drive will show if the valve is at fault. Also, you need 18" of mercury or more vacuum at the vacuum pump.