Hi

I own an Imprezza WRX 2001 (Bug Eye)and have a little issue which I need help for.

When the car is accelerating in first and second it's fine then in 3rd at about 3750 to 5000 rpm it doesn't run smoothly....it starts struggles to accelerate further (although it does..given about 10 seconds). Any ideas on what it could be?
Someone has suggest possible a new temperature sensor, asit does get slighlty better when warmer?