Five months after Secretary of State George P. Shultz instituted a plan to put several new faces in key embassy and policy posts, conservatives are still fighting a rear-guard action against what they charge is an "ideological purge" designed to give Shultz complete control over the administration's foreign policy machinery.

The latest indictment of how Shultz has allegedly become the captive of Foreign Service "elitists" was handed up Wednesday by Rowland Evans and Robert Novak, whose syndicated column has served throughout the controversy as a chart of conservatives' fever.

Their column was devoted to a new reminder that Senate and administration conservatives are opposed to Shultz's plan to name Rozanne L. Ridgway, a career diplomat who is ambassador to East Germany, as assistant secretary for European affairs. Richard R. Burt, who now holds that post, is scheduled to become ambassador to West Germany.

According to the column, Ridgway's opponents argue that she lacks experience in arms control and Soviet affairs, both subjects that have moved to the top of the foreign policy agenda. However, as Evans and Novak acknowledged, Ridgway's real sin seems to be that she served as State Department counselor during the final months of the Carter administration.

The columnists went on to charge that Shultz recently had put through "an unannounced regulation" that "limits political ambassadors, as contrasted to those from the Foreign Service, to a single 2 1/2-year term." They added that the rule "shows how thoroughly the Foreign Service bureaucracy has convinced the secretary that foreign policy should remain the province of an elite branch of government."

At the State Department, where the conservative charges have been a source of consternation and bemusement, senior officials said that the columnists apparently were referring to a plan devised some time ago by Ronald I. Spiers, undersecretary for management, for systematically rotating most U.S. ambassadors, whether career officers or political appointees, at three-year intervals.

"There is a general three-year time frame for both career and noncareer ambassadors' tours of duty," department spokesman Bernard Kalb said yesterday. He added that both State and the White House think this is a good idea, and he noted:

"Such a policy allows the president the necessary flexibility in the foreign policy field to make and change career and noncareer ambassadorial appointments as the situation requires. Of course, it is the president's wishes and decision on the nomination and tenure of ambassadors who are his personal representatives."

Ever since the charges of ideological warfare began late last year, Shultz's aides have argued that the secretary's personnel changes are designed only to carry out Reagan's policies more effectively. The officials Shultz removed did not meet his standards of job performance, these sources say, and those named as replacements were chosen not for their ideological credentials but because Shultz thinks their track records indicate they will perform well in sensitive diplomatic posts.

Kalb, who had been a network television correspondent, recalled how he decided late last year to leave the diplomatic reporting beat and called Shultz to say goodbye. The secretary unexpectedly responded by saying: "Bernie, if you're looking for a new challenge, why don't you come to work with us in the department?"

Similarly, Shultz reportedly picked Abraham D. Sofaer, a former U.S. District Court judge from New York, to become the department's legal adviser because he was impressed by the way Sofaer had presided over former Israeli defense minister Ariel Sharon's complex and emotional libel suit against Time magazine.

Although Reagan has publicly endorsed Shultz's style of personnel management, the unrelenting conservative opposition has delayed some appointments for months. Department officials nevertheless insist that the secretary "intends to be in command at State" and has no intention of bowing to demands that he apply an ideological litmus test for appointments.

These officials predicted that the upcoming shuffle of European posts will see Shultz prevail once again. They insisted that Shultz is so confident that Ridgway will become assistant secretary that he already has chosen Francis J. Meehan, another career diplomat who formerly was ambassador to Poland, to replace her in East Germany.