hi i have a peugeot 205 with a 1.9 mi16 conversion

since i got it the running temp has always been realy low but recently it has started to splutter about the 3500k mark in every gear part from first
was like overfueling. i put a thermistat in it because There was not 1 in it. and now it runs realy well when cold and mid temprature but when it gets hot it starts to lose oil pressure then were back to spluttering and chuging again checked the oil in it there is plenty but it is as thin as water so ive got some 15w40 there so im gona put that in it tomorow to see if it will hold oil pressure when hot.

if it is not that does any1 have any other suggestions as to what it could be any ideas would be good

cheers craig