Why do my car slow down going up hills.and go normal on straight roads

Repair your vehicle right away. Just click on the link I attached 2001 2003 Mitsubishi Lancer Evolution 7 EVO VII Service Repair Workshop...

timing may be off, the belt for the engine may be lose and need replaced, or you have a vacuum leak.

Hi,
The air filter can sometimes be a good place to start, as a slightly blocked/dirty/damaged one can give the car near normal operation on flat roads but when going uphill make it slow right down.

would suggest you try that first. If that doesn't work it could be a faulty/blocked fuel filter or dodgy turbo or slipping clutch or air/fuel mix issue (faulty sensor).

Check fuel pressure. Check for restricted exhaust system, Cat converter.

Yep that\'s what auto trans do. it tries to do engine braking, just don\'t accelerate at all. When you go down hill in a manual gearbox, you would also have to downshift to let the engine do the braking.

not getting enough back pressure in the exhaust system. weak or lose of power can be caused by this. usually middle box. if you hit it and it rattles thats were your problem is. also can be piston rings falling under pressure.

Not sure I am following your story.
Are you saying the transmission is slipping and therefore the RPM is high even tho the road speed is not ?
Or is the transmission not upshifting to the next gear at a normal road speed ?
Is the check engine light on ?

I have a 98 punto sporting nd today it started running like it was misfiring but wen i accelerate slow its fine but wen u accelerate as normal it spits and splutters :'( any suggestions??i have change fuel filter nd injectors ????

Better check the water separators and fuel filters. Also may have lift pump going bad

Is it a standard or automatic transmission? If standard, your clutch may be slipping. While driving down a straight road, push the brake while still in gear and push the gas pedal as well. If the engine slows with the vehicle, the clutch isn't slipping. If the engine speeds up while the vehicle is slowing down, the clutch is slipping.
If it is an automatic, check the transmission fluid. You may be low and the transmission is slipping while under load, or the torque converter is slipping for the same reason. You may have a bad torque converter clutch switch, but the computer should have picked that up and made your "Check Engine" light come on.
These are the main reasons you would be losing power going up hills unless your engine isn't powerful enough to handle the larger tires on extreme hills. (It takes more power to turn a larger wheel going up hill since the vehile has to travel further for each revolution of a larger tire. Incidentally, have your speedometer checked. You will be going faster than what the speedo says since you put larger tires on.) Good Luck.