Ring Digger

my RX 400H has no heat. There is no obvious leak from radiator or coolant as Ive had pressure tested at 3 different garages. However Im topping up very frequently but only on occassion because sometimes coolant reservoir is steady which is confusing. My car overheats sudden high speeds and cools down at constant fast speed on motorways. In the city on constant mid speed it overheats despite having coolant. uphill on a motorway it cools down. no garage i have approached including the main dealer have an answer other than to throw money at it to investigate the cause. my car is 2008 and drives great but is not worth throwing money at. Any answers or solutions?