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how does it idle? Is the idle smooth, or does it seem rough or unstable?
Well, if it were an engine related issue then idle quality should be affected here. If the cam timing was incorrect due to the belt being installed incorrectly the idle would surge. If the new valves were leaking, or if one of the old ones was damaged and missed during the repair, it would cause the idle to be rougher than normal. So, since teh idle quality is OK it is likely that the engine is operating normally.
All of the symptoms you describe are consistent with a fault related to the transmission. Why they are suddenly evident after an engine repair is anyone's guess... it could be that something transmission related was disturbed during the engine repair, but it is possibly coincidence.
Is your "check engine" light on?
If the check engine light was on, that would be an indication also that the engine was running abnormally, but since it is not that indicates the engine is not likely to be the cause of the problem here. If the light is not on, there will be no fault information stored in computer memory.
From the description, it sounds as if your problems are originating in the transmission. I would suggest starting by checking the fluid level. IF the fluid is full, then you will need to take teh vehicle in to a mechanic that perfroms transmission disgnosis and repairs to be checked (this is a specialty that not all shpps do. You may want to seek out a transmission specialty shop, or try the Mitsubishi dealer which will ahve a trained transmission tech on staff). Checking the transmission for proper operation will involve perfroming some hydraulic testing (connecting a hydraulic pressure gauge to verious pressure ports on the transmission to check to see that internal mechanical components are getting the proper pressure to actuate) and also using scan equipment to check transmission control system operation. Since this type of testing does require special test equipment to check to see where the problem may be occurring, it is not really something you can check or repair yourself at home in the driveway (other than checking to make sure the fluid level is full).