Heat exchanger help?

[Badburn]

Anyone have any experience with marine heat exchangers? My motors are getting hot if I run them above 3000 rpm for more than about 3-4 min. Temp comes down to normal if you reduce the rpm to 2000 or so. I already put new impellers in the raw water pumps............

[quiet riot]

T-stat sticking restricting flow? Circulating pump going bad? Some other restriction on the coolant or raw water side (manifold rusted, low level?) Pulling the t-stat would be the easiest and warming it up in a pan of water and see how far it opens and at what temperature (use a meat thermometer.)
Have the motors been bumped up in hp significantly from stock? Heat exchangers are sized for power output (ie heat) and if your making too much heat they may not be able to keep up. Is this a new issue that happened all of the sudden or after an upgrade? Or after running in some low water conditions?
Part of an impeller blade lodged somewhere in the cooling system on the raw water side? Were any blades missing on the old impellers? Heat exchangers can fill up with sand/sediment over time and need the end caps removed to flush the garbage out of the raw water side.
jd