1988 15 hp runs great until I run at idle for 20 min. then upon throttle-up, takes off fine for few seconds then dies down. If throttle held open, motor surges on and off. Have had it to the shop 3 times, they cannot replicate the issue in a tank. Carb has been re-built, new fuel pump, new gas tank, tried different tank hose. Any suggestions out there. Motor is in great shape for 1988.