2 NPN Transistor Square Wave Oscillator?
The reason for the sawtooth shape at the collectors of the transistors is caused by the B-E junction of the transistors and C1 / C2.

You can insert resistors in series with the base of each transistor - I'd start with 10k and see what happens.  Be sure that the resistors are in series directly at the base of each transistor.  That should give you a much more square-looking waveform at the transistor collectors.

The reason the sawtooth waveform happens is that the E-B junction of the transistors looks like a very-low impedance when the capacitors are charging via the 1k collector resistors.  Think of it this way: the capacitors are almost in parallel with the collector E-C leads.  There is a 0.6V region when the capacitor is beginning to charge that is high-impedance but as soon as the E-B voltage reaches 0.6V, the impedance drops to a very low value.