I have been battling a cooling issue with my recently finished roadster since last summer that I cannot figure out. Here's the details: 32 Roadster with .030 over 327, flat tops, 268 Comp cam, double hump heads and rochester FI unit. I know the mixture is correct, set with air/fuel meter and timing is correct. Walker Cobra radiator, zip riser, flowcooler high flow pump, 17" flex fan, currently no shroud. T5 with .63 OD and 4:10 gear. Have a brand new SW mechanical gauge and checked in boiling water to confirm working OK. Without a thermostat in the system and 50/50 mix last summer in 105 deg Texas heat I could not get the temp above 140 and maybe up to 150 when idling. Wanted to get the temp up to at least 180 so put a stock 180 deg thermostat in. Immediatly started having problems. Currently have a 170 Deg balanced high flow type thermostate in the system and checked it on stove and it is opening correctly. Here is what happens. As long as I am in 4th gear and turing around 2200 RPM everything is fine, temp is around 170/180. Go to 5th and RPM's drop to 1700 at 70MPH and temp starts climbing immediatly and keeps climbing to 240 withing a couple of minutes. Go back to 4th and temp drops right down to 180. Also having issues at idle, temp climbs to 230 or so quickly. This was yesterday in 60 deg weather. I tried changing to a stock pump with a cast iron impeller but it did no good. I know a shroud would help at idle but do not see where it makes any difference at 70 MPH. I have bled the Zip riser and raised the front of the car etc trying to make sure there is no air in system. I feel it is a water flow issue as it only gets hotter when the RPM's are down. Because it cools too good without the thermostat, how is the thermostat affecting the system? Any ideas would be appreciated.