I have a hyindai sonata 1990 it will start but when i give it a bit of a go it the revs will drop and wont kick back in for a few seconds it dose it when i come down gears some times when the revs drop it will stall while the car is still going about 50-60 km and back fire and will be difficult to start its good on a long trip but when it around town it just runs up to ****

could be one of a few reasons , get the OBD codes read , they will tell you which sensor/switch circuit is causing the issue

When was the last time you changed the air filter ?? Do you pour a bottle of fuel injector cleaner once a month into the fuel tank ?? Clogged dirty fuel injectors will give the symptoms you describe.

Mine did the same, replace kickdown servo. if not the problem, replace tranny.

tranny's gone

under the throttle body is the cold start unit where two hose pipes are and the one works as an inlet with first start with cold and after with hot water feeding and the other as out hot water return to engine. The job of that unit is to get the water temperature and as electronic connected with the ecu is giving the message for cold start rev up until the water temperature change to hot and revs dropped to the correct according to ecu. most propably is either one of the two hoses leaking or the flange itself.

have the fault codes read first to determine the problem area

Its fine, temp needle going up shows that engine is warming up and as soon as the radiator fan starts, temp of coolant drop and this is shown by the temp gauge needle. In most cars (even mine) it stays in middle most of the time unless you are in a traffic with AC on, it will show up more temp. What you can do is get the radiator flushed and coolant changed. Also check the water pump. To do so when engine is cold (in first start in morning), open the radiator cap and ask someone to rev the engine. If the coolant level moves up and down then water pump is also fine.

dear friend-when your vehicle wont rev it means that it went into limp mode and by disconnecting the battery you reset the ecu and will appear ok till the faulty sensor comes up again,have a dianostic test carried out to sole,previble when in limp mode.

Hmm, it's hard to diagnose without being there in person. Whatever help I can give you is this: The 9 causes of why an engine does not exert as much power as it should to take off right away as soon as you press down:
http://yourhandymanzone.com/Your_Handyman_Zone_How_To_Pages_Vehicle_Car_Zone_Cars_Trucks_Vans_Engine_System_Engine_Power_Less_Than_Expected_Normal.htm

Good luck!
Brad

Quick and easy attempt to fix might be to try some fuel injector cleaner,the purple bottle stuff I put in, is only about $3 a bottle, and seems to eliminate similiar symptoms brought on by bad gas.