It sometimes doesnt have enough power to run when I shiftinto 1st

That is near the border of the shift point. You may be able to influence it with accelerator or throttle positioner adjustment. That is only 49 mph.

Once again, not enough information given. How many miles on unit? When you say does not downshift do you mean when you press hard on accelerator pedal, or when you slow down? When you say does not shift into overdrive do you mean high gear or torque converter does not lock? The cars are know for transmission problems and while sometimes you get lucky, usually they require a rebuild or replacement. How often do you replace the fluid and filter?

I have a similar problem with mi Jetta 2010, I when the last week to the dealer and they didn't find anything bad, but the problem still hapenning.

Worn spark plugs, low spark power are a couple of possibilities but the demand for fuel is great when accelerating in the mid-gears and if the fuel system can't provide an adequate supply at the correct pressure during those conditions the engine performance will go flat.

Clearly the fact the engine light is on must be investigated, beginning with a code read. When the engine light is on the management computer has detected a problem that could affect the exhaust emissions and has switched to pre-programmed values designed to get you to the nearest dealer, or get you home.

It is always best as part of the diagnostic process to carry out any basic and routine maintenance that is due plus any other checks that might reveal pending problems that might cause further unreliability.

The shift linkage may need rebuilding. Inspect it. Jetta 3 2 0L 8V 9

reverse is 1st and 3rd gears its the highest pressure in the trans if its weak so is either 1st or 3rd gear but not both

the clutch may burn out on those gear, check the fluid if it smell burn

Volkswagens are known for having trouble with their transmissions, even more so in the 1st couple generations. This problem could just be your transmission sticking when getting into another gear. I recommend not pushing the car so your running at high RPMs, slowly work your way up that hill, and even try letting off the gas it may help it shift some. If you're really concerned with it you can take it somewhere to get the transmission checked, something maybe slipping or sticking it all depends and it can be a small inexpensive fix or it could be a heck of a lot, but its all up to you.

you do mean its a 5 speed with a clutch right? If so you need to purchase a heavy duty clutch which should handle it well .. that is if its a 5 speed standard shift... holler if you need more Jerry

YOU NEVER SHIFT INTO 1st GEAR!!