It's been two months now since the Reagan administration was supposed to be ready to "hit the ground running" and more than four months since President Reagan was elected with a solemn vow to end the "zigzagging and vacillation" in American foreign policy.

Finally, this country's allies and adversaries would hear one, clear voice. There was to be no more of this business of sending mixed signals that had so eroded the prestige and power of the United States in the world. So what has there been?

The performance, zigzagwise, has been almost ludicrous, even granting that it is early on; key departments are still shot through with unfilled policymaking vacancies; the whole process is still a long way from having shaken down. You could make a case, on the face of it, of incompetence and incoherence beyond remedy.

But when you examine the zigs and zags closely, you can make a stronger case of an administration, for better for worse, struggling to work its way free of antediluvian dogma and case-hardened arch-conservative doctrine -- trying to come to terms with itself and the realities. Examples:

Zig: Secretary of the Navy John Lehman takes it upon himself to propound publicly the argument that there is no legal reason for the United States to honor the provisions of both the expiring SALT I and the unratified SALT II arms control agreements. He would be against doing so, even by tacit arrangement with the Soviets.

Zag: Rightly reading this as elbow-jostling from the far right at a delicate moment in the administration's review of arms control strategy, and mindful of European sensitivities, Secretary of State Haig swiftly repudiates Lehman by name. An unusually blunt State Department statement, rushed to reporters, says Lehman's remarks "were not authorized, nor did they reflect administration policy."

Zig: Secretary of Defense Caspar Weinberger announces that he is in favor of reviving the idea of equipping American missiles and artillery shells in Europe with "enhanced radiation" warheads (otherwise known as neutron bombs).

Zag: Keenly aware again of how touchy the issue is in Europe, and of where it fits into larger alliance defense planning, Haig wisely takes steps to make clear to our allies that this is by no means a final decision.

Zig: With the most elaborate orchestration, "war-torn" El Salvador is elevated to the pinnacle of East-West concern. The State Department speaks of "massive" arms flows from Cuba. Haig warns of "striking at the source." The threat, Soviet in origin, is said to be hemisphere-wide. All Europe, and all Latin America, must join in drawing the line. The U.S. Military adviser force is doubled; aid is tripled. This is war.

Zag: The president allows as how there was no real thought of "striking" at Cuba. Political solutions, and the importance of economic help and an end to repression by the U.S.-backed Duarte government, are given equal importance with military measures. Almost laughably, a State Department official complains that the whole story has been overblown by the press. It was only supposed to be a "signal" that, once delivered, should now be turned off. Be good enough, the press was told, to turn your attention elsewhere.

Zig: The secretary of agriculture announces his undying opposition to the embargo imposed by Jimmy Carter on grain shipments to the Soviet Union.

Zag: After examining both the effectiveness of the embargo and the effect of lifting it in return for nothing, the administration sensibly puts that decision on hold.

In extenuation, it quickly be said that these (and other) zigs owned much to preconceptions made plain, and promices firmly offered, in the course of the Reagan campaign. The president did say that the United States should not abide 'by SALT II's terms "prior to ratification." He did favor the neutron bomb, oppose the grain embargo and warn of falling "dominoes" in Central America -- with Mexico and, ultimately, the United States, the last in line.

To some part of the problem has to be that the administration's top figures hit the ground still running for president, so to speak, still faithful to every jot and title of the Reagan line. Some part of it has also to do with the president himself. Left to his own devices (in the interview with Walter Cronkite), he betrays a sort of fierce fidelity to a lot of his much earlier thinking, without much recognition of what may be new and different about today's Cold War.

And some part of it has to do with a sort of "scorched-earth" approach by long-frustrated conservatives finally come to power and determined to erase every fingerprint of the recent past: the Law of the Sea, the Carter refusal to upgrade Saudi Arabian F15s and -- above all -- human rights.

There remains that part of the zigzagging that has to do with getting organized: the system, quite obviously, is not firmly in place. Meantime, there is one thing to be said for the Reagan vacillation: The second-thought zags have in every instance been an improvement over the zigs.