Could be any number of things - Intake screens clogged up, t-stat not opening all the way, bad impeller in the water pump, worn impeller housing, blocked water passage, or a faulty pressure relief valve. Do you have a Helm shop manual? If so, there is a test in there for the pressure relief valve. If that tests out, then you need to go on a bug hunt. Start with removing and checking the intake screens on the lower unit. Then move to the t-stats and test them by heating them up in hot water and measuring at what temperature they open and how far they open IAW the owner's manual. If all that tests out, then you will need to drop the lower unit and inspect/replace the impeller. Autually, it's probably better to replace the entire water pump, it's not that expensive and not much more than the impeller kit.