This is probably because you don't have enough TWR to go vertically so you lose speed and stall. Maybe give us some more info about it (how fast it goes straight up, its purpose etc.) so we can help you more? Until then, the best thing to do is to increase the TWR one way or another by, for example, switching out the engines (I would replace the rapiers [the rapiers are only really useful for SSTOs; they provide little to no advantage to atmospheric jets and even have less thrust than the turbojet] with basic jet engines if you're planning to fly low in the atmosphere or turbojets if you want to be flying at high altitudes. If you are making an SSTO, try separating the jet engines and rocket engines.), decreasing weight, or by simply not climbing at too steep of an angle.