Power Loss While Driving

[odofin]

HI, MY TRANSIT IS A 2.0 DOHC PETROL/GAS, I NEVER REALLY USED THE GAS I ONLY DRIVE ON PETEROL, I WAS DRIVING ON THE MOTORWAY FEW DAYS AGO WHEN I NOTICE IT STARTED TO LOSE POWER, I WAS JUST ABOUT TO CLIMB A LOW HILL, I CHANGED GEAR FROM 5 TO 4 AND IT CONTINUED FOR FEW MINUTES AND STARTED TO SLOW DOWN AGAIN AND WAS GETTING REALLY BAD SO I PARKED TO FIND OUT WHAT WAS WRONG, THERE WAS A SMALL WATER LEAK RIGHT IN FRONT OF INLET MANIFOLD, WATER LEVEL WAS STILL OK, AFTER ABOUT 5 MINUTES I STARTED ENGINE IT STARTED OK AND UP I WENT FOR ANOTHER 10 MILES WHEN IT STARTED AGAIN, IT GETS WORSE WHEN CLIMBING HILLS, I MANAGED TO GET IT HOME AND I WAS ADVISED TO GET THE WATER LEAK SORTED, WHICH I DID, THERE ISN'T ANY MORE LEAK BUT THE POWER LOSS CONTINUE, I DROVE IT YESTERDAY FOR OVER 20 MILES BEFORE IT STARTED, ONCE YOU PARK IT FOR SOME MINUTES IT WILL BE FINE FOR ANOTHER 10 TO 20 MINUTES, ANY ADVICE WHAT THE PROBLEM COULD BE? CHEERS GUYS