Pronunciation problem with "park" and "walk". Is there a name for this "phenomenon"?

The technical term is Dissimilation, usually. There are more technical variants, depending on the details of individual variations and habits. Dissimilation is an opposite of Assimilation; both are Latin words, describing the processes of (respectively) 'becoming less similar' and 'becoming more similar'.

Similar in sound, in this case. Both processes are normal phonological phenomena, due to the realities of pronouncing certain sequences of sounds, and both are often the source of historical changes.

Assimilation is the reason why there's a P in assumption, for instance — the voiceless dental stop /t/ (back in Latin, where it was actually /t/ instead of /ʃ/ like English) assimilates its beginning to the preceding labial nasal /m/, producing an unavoidable voiceless labial stop /p/, whether it's spelled or not.

Dissimilation, on the other hand, changes one or more of several identical sounds that come close together but are separated by other sounds. Phonemes like retroflex /r/ and lateral /l/, which are similar in production, are difficult to pronounce in sequence but easy to alternate between, so they are particularly prone to this in European languages:

• the pronunciation of colonel dissimilates the first /l/ to an /r/ (but keeps the spelling).
• marble comes from Latin marmora, where the second /r/ has dissimilated to an /l/ (and the second /m/ has dissimilated to a /b/).
• The German verb 'to murmur' is murmeln, where the second /r/ has dissimilated to /l/.

Then there are all kinds of productive dissimilations, like

"Factors, schmactors! What's the bottom line?",

or dissimilative reduplications, like pitter-patter, mishmosh, or repple-depple; and even fully-dissimilated (though no longer productive) derivational morphemes, like the adjective-forming suffix in this puzzle.

This is a frequent type of mispronunciation, where one speech sound occurs too early or too late. If a sound is produced too early, i.e., the error occurs before the source, then it is called an anticipatory error ("pork and walk", anticipation of vowel sound). If the error occurs after the source, then it is called a perseveration ("park and wark", perseveration of /r/). These errors typically occur if the source and error sounds and syllables are phonetically similar, i.e. they share several features and differ in only a few features, and if the two syllables involved are not too far apart (a span of 7 syllables has been suggested) as in your examples.

See http://en.wikipedia.org/wiki/Speech_error and references listed there.