Juddering, loss of power

hello
i hope somebody can help me please ?
i own a mitsubishi l200 warrior 2003, nearly 3 months ago ,i had another engine fitted,as the previous engine blew up.
since having the other engine fitted ive had no issues until recently ,when driving along got a slight glitch which lasted about a second then cleared,the rev counter flicked up and down randomly,then went on as normal as nothing happened.
i drove over 125miles with no fault in one stint,until i was 4miles away from home it started juddering abit,rev counter going awol,engine management light came on momentarily then went out again.
the engine starts fine and runs,the turbo kicks in ok.
when it does the glitch,and juddering i was doing about 50mph,when it starts if you put your foot down on the accelerator,you dont have any power,but if you take your foot off when it glitches then re apply the accelerator it would clear itself and be fine again.
because it was an intermitant problem the garage couldnt find anything.
so i took it for a short drive the next day,and the juddering and glitching started and it got worse and didnt clear when i tried to do what i previously done with the throttle pedal. the engine management light popped on and went out again,a puff of white smoke came out of the exhaust,the rev counter went berserk and had loss of power altogether.
i got it recovered to my nearest garage, the engine starts ok and runs and the turbo is working too,ive had a diagnostics test done on it,and it came up with boost sensor and throttle solenoid faulty ,i have had these 2 problems sorted out and replaced.
the mechanic took it out on a test drive today and he said it drove perfectly for half hour with no problems at all,then it started glitching again as if its something making it do it when the engine warms up. the temperature gauge on vehicle is working at the right temperature and has done throughout.
i would appreciate anybodys ideas and thoughts on this problem.
MANY THANKS