Ok. Smooth idle and power brakes now but hard start & stumbling takeoff? Sounds like its running on less than 6 cylinders under load, probably spark plugs or leads. Change the plugs first then work on the leads. *turn engine off* before touching the spark plug leads - these are *high* voltage.
Replace the plugs - make sure to use a correct or equivalent plug set to the same plug gap. The plugs in it might not be the correct ones, and looking at them will tell you a lot about each cylinder - oily/sooty or nice & grey?
A USA 2.5 owner or shop should be able to advise which plugs are best value over there. Go gentle tightening them into the alloy head, it's not strong like a cast iron head and if the thread strips youve got big trouble. The usual rule is hand tight (to the bottom of the plug thread), then a quarter turn or so just to compress the new washer. Technical types do it with a torque wrench. Ask a friend with alloy head experience to help.
Then make sure the top of the plug connects properly to the lead, connectors are not rusty but shiny(both ends), the leads are clean, clear of the engine (not rubbing or shorting) and plugged into the top of the distributor (dizzy) properly. Do one at a a time. Check the coil to distributor lead too.
Pop the top off the dizzy (undo the clips first
) and clean the plastic cap and rotor button in there, don't scratch it. The carbon post in the middle of the dizzy cap is easily damaged, dont touch it. Look for black lines where the spark might be tracking from a lead connector post to earth. Reassemble as found. Leads *must* join same plug to same distributor cap hole, maybe number dizzy posts with liquid paper pen or photo or both.
On my V6 the leads were cooked, (that damages the insulation and then they short out) brittle and the only choice was to replace them.
If moving them around makes a bit of a difference, its probably time to find some new ones. Modern leads have a graphite conductor which is easily damaged, so treat them kindly. If they are kinked or stretched the conductor cracks.