Learn English – “I wish it is just sometimes.”

You can't use your sentence as you wrote it. But there is a context in which a construction like yours would sound idiomatic in English. If someone has just said to you something along the lines of:

Wow, that's crazy. I guess that kind of thing happens sometimes in your line of work?

You can then respond:

I wish it were only sometimes.

or:

I wish it were just sometimes.

Note the subjunctive mood--"were" or colloquially, in some dialects, "was"--this is mandatory.

You can even provide the necessary context yourself--but you have to do it explicitly:

They told me when I took this job that it would make me crazy sometimes. Hell, I wish it were only sometimes.

Your phrasing is going to sound awkward in just about any other context. However, depending on the context you can express the same idea in slightly different words:

You're going to see patients who would deny drug use even if the needle was still sticking out of their arm. And it's not just going to be sometimes.

or

I've seen patients deny drinking a drop of alcohol even though their breath was so strong it got the duty nurse drunk. I wish I could say it only happened a couple of times.

or

... I wish I could say it was only once in a while.