Overheating worries

[deco]

My 1995 300Tdi Auto is overheating when towing my horse trailer up hill. On level ground it's ok and fine on it's own. I recently replaced expansion bottle due to split in side but it was overheating before this. However since replacing the bottle i've notice the transmission temp light coming on at the same time as the engine is overheating. As soon as i reach the top of a hill or ease off the gas the temp returns to normal within a couple of minuets. Can any one give me ideas that don't cost an arm and a leg. Thanks