overheating

hi i have a p reg bmw 1.8i e36 with overheating problems, the oil and water are both fine, so no sign of head gasket problems, i hope, and also no hot air comes though air vents, also when running for around 5mins the hose from the water tank to the head becomes rock solid, very hard, i did break down on the m25 as overheated and AA came, he said there was an air block in the system, he sorted it out and got the car home fine but within 3days overheated again, i gotta no the problem as dont wanna waste money on changing head gasket , thanks paul