Why do I sometimes need starting fluid to get my 94 Jetta to start?

Some history on what I've done to the car so far: Replaced spark plugs with Bosch 4301's Replaced exhaust system with aftermarket no-name cat-back Replaced battery before winter 09 Problem: 95% of starts are fine. 100% of the time, when the car is started, it runs without issues. I did have a... show more Some history on what I've done to the car so far:

Replaced spark plugs with Bosch 4301's
Replaced exhaust system with aftermarket no-name cat-back
Replaced battery before winter 09

Problem:

95% of starts are fine. 100% of the time, when the car is started, it runs without issues. I did have a stutter problem when hitting the accelerator off the line, but that was fixed with changing the plugs.

In the few cases where the car doesn't want to start, spraying some starting fluid into the air intake housing allows the car to start without incident.

Conditions:

It's never the same weather conditions when the problem occurs. Sometimes it will be cold, other times warm, sometimes it will rain, other times it's dry. The problem has happened during 3 different seasons so far (fall, winter, spring).

When it happens, it can also go for months without happening again. The engine always turns, I've never had just the starter turning, so I don't think it's the starter.

Speculation:

I'm assuming that it's not the starter because it's such an intermittent problem and the starting fluid fixes the issue.

Also assuming it's not a faulty connection in the ignition coil for the same reason above.

I don't have any acceleration problems once running, so I doubt it's a fuel pump issue.

Could it be the fuel filter?

Maybe the air filter needs to be changed? (but it doesn't look dirty at all)

Should I just change the stock intake altogether? (it is pretty old, after all)
Update: Are the vacuum lines easy to change?

Meaning, are they just buy new ones and replace or do I need to do anything else to prime them before I put them on?

I'm pretty handy, but I'm definitely no car expert :P
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