Classic multivibrator circuit and LTSpice, why is there reverse voltage on base even with diode?
In the schematic below, I decided to include fairly common high-current BJTs as your lamps, when cold, will have very low impedance and will incur high-currents at the outset until they warm up, sufficiently.



simulate this circuit &ndash; Schematic created using CircuitLab
Please take note of how I arranged the base-emitter protection diodes. This should clamp the base voltage and protect the BJTs there. This is different from your arrangement. Because of this arrangement and the speed by which this allows capacitor discharge, larger capacitors are also required. I assume you don't mind.
In LTspice, I recommend the following .ASY file for the lamps:
Version 4
SymbolType CELL
LINE Normal -12 -32 -17 -44
LINE Normal 28 -32 33 -44
LINE Normal 6 -32 2 -59
LINE Normal 10 -32 14 -59
RECTANGLE Normal 29 -4 -13 -32
RECTANGLE Normal 3 0 -3 -4
RECTANGLE Normal 19 0 13 -4
ARC Normal 1 -71 7 -57 7 -64 -1 -58
ARC Normal 5 -71 11 -57 11 -64 6 -64
ARC Normal 9 -71 15 -57 17 -58 10 -64
ARC Normal 7 -57 5 -71 5 -64 7 -64
ARC Normal 11 -57 9 -71 9 -64 11 -64
ARC Normal -24 -96 40 -32 33 -44 -17 -44
WINDOW 0 -13 -111 Left 2
SYMATTR Prefix X
SYMATTR Value incandescent
SYMATTR SpiceLine Pn=20 Vn=12
SYMATTR ModelFile incandescent.sub
PIN 0 0 NONE 8
PINATTR PinName 1
PINATTR SpiceOrder 1
PIN 16 0 NONE 8
PINATTR PinName 2
PINATTR SpiceOrder 2

Copy that text and save it into your ../lib/sym subdirectory as &quot;incandescent.asy&quot;.
Separately, you'll also need a model. For this, use the following:
* Pn: Lamp Power
* Vn: Lamp Voltage
* Tamb: Ambient temperature (default in LTspice: 27 C)
* Kamb: [Hot resistance / cold resistance] factor
.subckt incandescent 1 2 params: Pn=20 Vn=12 Tamb=25 Kamb=16 CTf=50m RTf=1k
.PARAM Rn={Vn*Vn/Pn}
.PARAM Ramb={Rn/Kamb}
.PARAM Tambk={Tamb+273.15}
.PARAM Tfilk={Tambk*(Rn/Ramb)**(1/1.2)}
.PARAM Kc={1/Ramb*Tambk**1.2}
.PARAM Kf={(Pn+(Tambk-Tfilk)/RTF)/(Tfilk**4-Tambk**4)}
BTf 0 Tfilk I={V(1,2)*I(BCf)-(Kf*(V(Tfilk)**4-V(tambk)**4))}
Cfa Tfilk Tambk {CTf} Rpar={RTf}
Va Tambk 0 {Tambk}
BCf 1 2 I=V(1,2)*Kc/V(Tfilk)**1.2
.ends

Copy that text and save it into your ../lib/sub subdirectory as &quot;incandescent.sub&quot;. If curious, you can look back at the .ASY file I show above and see that there is a line where it says &quot;SYMATTR ModelFile incandescent.sub&quot;. That's the filename you need. You can change the filename if you want, of course. But these need to match, because the symbol must point to where to find the .SUBCKT model.
You should now have a lamp model and schematic image you can apply to your schematics.
One more detail. If you look above, you will see &quot;SYMATTR SpiceLine Pn=20 Vn=12&quot; as a part of the symbol. You will also see &quot;params: Pn=20 Vn=12 Tamb=25 Kamb=16 CTf=50m RTf=1k&quot; as part of the .SUBCKT line. This is just the default that you can override.
In your case, you want to override them. Your lamp should read &quot;Pn=2.4 Vn=6&quot; as it's a \$6\:\text{V}\: @ \:400\:\text{mA}\$ device. The Pn variable is the power and the Vn variable is the designed operating voltage. So when you place down the incandescent symbol on your schematic, you'll need to right-click the lamp symbol and edit the &quot;SpiceLine&quot; entry and modify it.
There's another value you can also modify: Kamb. Incandescent lamps have a cold resistance and a hot resistance. This is the ratio of the hot resistance divided by the cold resistance. By default (please read above to see it), the value is Kamb=16. This may not be accurate. Many devices may be 6 or 8 or something less. So you can also modify the &quot;SpiceLine&quot; entry and add that to it to change it as you feel is appropriate. For example, you might use &quot;Pn=2.4 Vn=6 Kamb=6&quot; instead of what's showing there. You can call the shots here.
Finally, when running the schematic with .TRAN I recommend using UIC and also setting the maximum timestep to about \$10\:\mu\text{s}\$. So here is an example:

It's output is:

You can easily see that as the lamp heats up, the current in the lamp declines and starts to approach the designated value for it. It doesn't actually reach it, because each lamp is cooling in between times when it is turned on. But at least this provides some kind of reasonable expectation.
Just as a note, though. I used \$470\:\text{k}\Omega\$ resistors because I worried a little about the BJTs finding a quiescent point instead of oscillating. (I think your choice of \$150\:\text{k}\Omega\$ pushes too close towards allowing a quiescent point. The Sziklai pair has too much current gain.) I might go as low as \$330\:\text{k}\Omega\$. But no lower than that.

You used series diodes to protect the base-emitter junctions of the NPN. That's fine. But then you probably should include a galvanic connection across the diodes, as well.
In this case, something like this:

The resulting simulation looks like this:

Try out both and see how it works for you.