Do not leave spice simulations running for unreasonably long times. This will store all simulation results in an output file named output.file, so you won't have to repeat the slow simulation more than once. If Spice fails to converge, and aborts with an error message, this does not necessarily mean that your circuit is wrong. Still, it is useful to first check your circuit connections. Allowing larger errors in simulation results may help convergence, and may speed up simulations.
Spice uses the larger of the specified relative and absolute tolerance.
If the simulation succeeds, try to use smaller values in order to improve accuracy of the results. If convergence problems occur, try to do just one analysis, to find out which one is the problem. You may still be able to generate useful results with other analysis types that do not have convergence problems. This tells spice to skip solving the initial dc bias point at the start of transient simulation, and instead start from initial conditions given by capacitor voltages. The uic option can save simulation time and help avoid convergence problems in transient simulations, especially with circuits that contain logic gates, flip-flops etc.