All things being equal, no, it's not good idea: people entering to visit girlfriends/fiances/etc for long periods of time are at high risk of overstaying and/or working illegally, and you'll get additional scrutiny.
However, it's much worse to lie about why you're visiting, since immigration officers are very good at spotting any inconsistencies in your story and you're basically guaranteed to be denied entry if you're caught. You also will be asked how long you're staying, and when you say "five months", they will ask a) why so long, b) how you're going to support yourself, and c) how do they know you will go back home.
So my advice would be to find another reason to go to the US and explain this when asked -- but don't lie, it has to be real. For example, maybe you can enroll in some local training course that will assist you in your career when you get back, and if asked where you're staying, you can state that you're renting a cheap room from/sharing a flat with a "friend" you met in the Czech Republic. (No need to mention that you're romantically involved, unless explicitly asked!) Remember that they will likely know that you've already stayed for five months before, so be sure your explanation covers that and that your girlfriend knows what to say in case they call her.
Note: This all assumes you did not overstay during your previous visit. If you entered the US using the Visa Waiver Program, you only get 90 days, so you may already be in hot water if you actually stayed five months. (You're not stopped on the way out, because the US doesn't have exit immigration control, but this is recorded.)