I have used this opening some times and I won.But sometimes I was almost destroyed.Is the move 6.Qd4 dubious or am I doing something wrong later?
[FEN ""] 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 g6 5.Nxc6 bxc6 6.Qd4 Nf6 7.e5 Ng8 8.e6 Nf6 9.exf7
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No, 6. Qd4 is not a dubious move. In the variation that you mentioned, the moves that are actually dubious are 7...Ng8 and 8. e6
8. e6 is dubious because it "forces" Black to develop his knight to a good square (!) and also gives up White's strong central pawn on e5, which could have hindered the development of Black's knight on g8. After 9. exf7+, it may seem that Black is robbed of the castling option, but that is just superficial. Black can easily develop his bishop to g7 and bring the h8 rook to e8 or f8 as needed. None of White's pieces can really exploit Black's king position on f7, so White gains nothing from giving up the e-pawn.
7...Ng8 is dubious because there is a better square for the knight - d5. Also, after 7...Ng8, White isn't forced to play 8. e6, so the knight will have to waste another tempo for development.
6. Qd4 is playable. It has been played by players of the calibre of former world champion Emmanuel Lasker and also Grandmaster Beliavsky. After 6...Nf6 7. e5 Nd5, the position is equal but playable for both sides.
[FEN ""] [White "6.Qd4"] [Black "Playable"] 1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 g6 5. Nxc6 bxc6 6. Qd4 Nf6 7. e5 Nd5 ( 7... Ng8?! 8. Nd2! (8. e6?! Nf6 9. exf7+ Kxf7) 8... Bg7 9. Nf3 d6 (9... f6 10. e6 dxe6 11. Bc4 Qxd4 12. Nxd4 e5 13. Nxc6 Bb7 14. Bb5) 10. Bf4) 8. Nc3=
'Dubious' is a vague term. Rauan's answer would seem to suggest that an engine would have little trouble defending the position as either side, but there's a little more to it than that.
Firstly, an engine being able to hold a position does not necessarily mean that it is not 'dubious' - it may be extremely difficult for a human to play without the resourcefulness of an engine, and probably means that you would be playing for a draw against a strong human.
Secondly, with the move Qd4 you have given up almost all of white's initiative because the queen will lose time once attacked. As white you generally want to be pushing for a win and be able to make as much use of the first move advantage as possible, but that advantage has just been lost. As expected, engine analysis goes from a (very) small plus for white to more or less equal.
Finally, Black has a much easier time finding moves from this position - his plan of developing pieces while gaining time where possible on the queen, then moving the kind to safety, making use of the half-open files and occupying the center gives him several options at every turn. By comparison, white will likely spend much of the next few moves reacting to black's choices while hoping that long-term kingside safety is enough compensation for the lead that black should get in development, central control and attack.
Objectively you are not worse in this position (engine searches eventually give a small minus score, but so small as to be negligible), but that does not mean that Qd4 was a good move when you had a small advantage beforehand. This is not to say that you shouldn't play it if you find that the resulting games are enjoyable, but it is hard to argue that it is objectively the right decision.
After a quick check, the final position after
[FEN ""] 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 g6 5.Nxc6 bxc6 6.Qd4 Nf6 7.e5 Ng8 8.e6 Nf6 9.exf7 Kxf7
is evaluated as equal by the engine (Komodo 6). Black has the king on f7, where it can become a target along the a2-g8 diagonal and by a white knight reaching e5 or g5. Yet, black is ready for d7-d5 and the white g1-knight has been exchanged off the board. Meanwhile, black has better control in the center thanks to the e7-pawn ready to push e7-e5 at some point.
Black has a more or less clear plan with Bf8-g7, e7-e5, Rh8-e8 and d7-d5. Black probably wants to exchange queens with Qd8-b6 at some point. While white has to be a bit more creative and try to launch an attack on the enemy king. Perhaps with Bf1-e2 and h2-h4-h5.
All in all, the position is interesting and rich in ideas. It seems fully playable for white as well as black.